[Sandler ONeill & Partners, L.P. Letterhead]

[Sandler O’Neill & Partners, L.P. Letterhead]

Exhibit 1.1

December 11, 2003

Board of Directors

SI Bancorp, Inc.

803 Main Street

Willimantic, CT 06226

Board of Directors

The Savings Institute

803 Main Street

Willimantic, CT 06226

Ladies and Gentlemen:

Sandler O’Neill & Partners, L.P. (“Sandler O’Neill”) is pleased to act as an independent financial advisor to the The Savings Institute (the “Bank”) and SI Bancorp, Inc. (the “Holding Company”) in connection with the formation of a middle-tier stock holding company (the “Reorganization”) and the offer and sale of certain shares of the common stock (the “Common Stock”) of the newly-organized middle-tier stock holding company (the “MHC”) to the Bank’s eligible account holders in a Subscription Offering, to members of the Bank’s community in a Direct Community Offering and, under certain circumstances, to the general public in a Syndicated Community Offering (collectively, the “Offerings”). For purposes of this letter, the term “Actual Purchase Price” shall mean the price at which the shares of the Common Stock are sold in the Offerings. This letter is to confirm the terms and conditions of our engagement.

ADVISORY SERVICES

Sandler O’Neill will act as a consultant and advisor to the Bank, the MHC and the Holding Company and will work with the Bank’s, the MHC’s and the Holding Company’s management, counsel, accountants and other advisors in connection with the Reorganization and the Offerings. We anticipate that our services will include the following, each as may be necessary and as the Bank, the MHC and the Holding Company may reasonably request:

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 2

FEES

If the Offerings are consummated, the Bank, the MHC and the Holding Company agree to pay Sandler O’Neill for its services hereunder the fees set forth below:

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 3

If (i) Sandler O’Neill’s engagement hereunder is terminated for any of the reasons provided for under the second paragraph of the section of this letter captioned “Definitive Agreement,” or (ii) the Reorganization is terminated by the Bank, no fees shall be payable by the Bank or the Holding Company to Sandler O’Neill hereunder; however, the Bank and the Holding Company shall reimburse Sandler O’Neill for its reasonable out-of-pocket expenses (including legal fees) incurred in connection with its engagement hereunder and for any fees and expenses incurred by Sandler O’Neill on behalf of the Bank or the Holding Company pursuant to the second paragraph under the caption “Costs and Expenses” below.

All fees payable to Sandler O’Neill hereunder shall be payable in cash at the time of the closing of the Offerings. In recognition of the long lead times involved in the reorganization process, the Bank agrees to make an advance payment to Sandler O’Neill in the aggregate amount of $25,000, which shall be payable upon execution of this letter and which shall be credited against any fees or reimbursement of expenses payable hereunder.

SYNDICATED COMMUNITY OFFERING

If any shares of the Common Stock remain available after the expiration of the Subscription Offering and the Direct Community Offering, at the request of the Bank and the Holding Company and subject to the continued satisfaction of the conditions set forth in the second paragraph under the caption “Definitive Agreement” below, Sandler O’Neill will seek to form a syndicate of registered dealers to assist in the sale of such Common Stock in a Syndicated Community Offering on a best efforts basis, subject to the terms and conditions set forth in a selected dealers agreement. Sandler O’Neill will endeavor to limit the aggregate fees to be paid by the Bank and the Holding Company under any such selected dealers agreement to an amount competitive with gross underwriting discounts charged at such time for underwritings of comparable amounts of stock sold at a comparable price per share in a similar market environment, which shall not exceed 7% of the aggregate Actual Purchase Price of the shares sold under such agreements. Sandler O’Neill will endeavor to distribute the Common Stock among dealers in a fashion which best meets the distribution objectives of the Bank and the Holding Company and the requirements of the Plan of Reorganization, which may result in limiting the allocation of stock to certain selected dealers. It is understood that in no event shall Sandler O’Neill be obligated to act as a selected dealer or to take or purchase any shares of the Common Stock.

COSTS AND EXPENSES

In addition to any fees that may be payable to Sandler O’Neill hereunder and the expenses to be borne by the Bank and the Holding Company pursuant to the following paragraph, the Bank and the Holding Company agree to reimburse Sandler O’Neill, upon request made from time to time, for its reasonable out-of-pocket expenses incurred in connection with its engagement hereunder,

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 4

regardless of whether the Reorganization or the Offerings are consummated, including, without limitation, legal fees, advertising, promotional, syndication, and travel, up to an aggregate maximum of $50,000; provided, however, that Sandler O’Neill shall document such expenses to the reasonable satisfaction of the Bank and the Holding Company. The provisions of this paragraph are not intended to apply to or in any way impair the indemnification provisions of this letter.

As is customary, the Bank and the Holding Company will bear all other expenses incurred in connection with the Reorganization and the Offerings, including, without limitation, (i) the cost of obtaining all securities and bank regulatory approvals, including any required NASD filing fees; (ii) the cost of printing and distributing the offering materials; (iii) the costs of blue sky qualification (including fees and expenses of blue sky counsel) of the shares in the various states; (iv) listing fees; and (v) all fees and disbursements of the Bank’s and the Holding Company’s counsel, accountants, conversion agent and other advisors. In the event Sandler O’Neill incurs any such fees and expenses on behalf of the Bank or the Holding Company, the Bank and the Holding Company will reimburse Sandler O’Neill for such fees and expenses whether or not the Reorganization is consummated.

DUE DILIGENCE REVIEW

Sandler O’Neill’s obligation to perform the services contemplated by this letter shall be subject to the satisfactory completion of such investigation and inquiries relating to the Bank, the MHC and the Holding Company, and their respective directors, officers, agents and employees, as Sandler O’Neill and its counsel in their sole discretion may deem appropriate under the circumstances. In this regard, the Bank and the Holding Company agree that, at their expense, it will make available to Sandler O’Neill all information which Sandler O’Neill requests, and will allow Sandler O’Neill the opportunity to discuss with the Bank’s, the MHC’s and the Holding Company’s management the financial condition, business and operations of the Bank, the MHC and the Holding Company. The Bank and the Holding Company acknowledge that Sandler O’Neill will rely upon the accuracy and completeness of all information received from the Bank, the MHC and the Holding Company and their directors, trustees, officers, employees, agents, independent accountants and counsel.

BLUE SKY MATTERS

The Bank and the Holding Company agree that if Sandler O’Neill’s counsel does not serve as counsel with respect to blue sky matters in connection with the Offerings, the Bank and the Holding Company will cause the counsel performing such services to prepare a Blue Sky Memorandum related to the Offerings including Sandler O’Neill’s participation therein and shall furnish Sandler O’Neill a copy thereof addressed to Sandler O’Neill or upon which such counsel shall state Sandler O’Neill may rely.

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 5

CONFIDENTIALITY

Other than disclosure to other firms made part of any syndicate of selected dealers or as required by law or regulation or legal process, Sandler O’Neill agrees that it will treat as confidential all material, non-public information relating to the Bank, the MHC and the Holding Company obtained in connection with its engagement hereunder (the “Confidential Information”) whether or not the Reorganization is consummated. As used in this paragraph, the term “Confidential Information” shall not include information which (i) is or becomes generally available to the public other than as a result of a disclosure by Sandler O’Neill, (ii) was available to Sandler O’Neill on a non-confidential basis prior to its disclosure to Sandler O’Neill by the Bank, the MHC or the Holding Company, or (iii) becomes available to Sandler O’Neill on a non-confidential basis from a person other than the Bank, the MHC or the Holding Company who is not otherwise known to Sandler O’Neill to be bound not to disclose such information pursuant to a contractual, legal or fiduciary obligation.

INDEMNIFICATION

Since Sandler O’Neill will be acting on behalf of the Bank, the MHC and the Holding Company in connection with the Reorganization and the Offerings, the Holding Company, the MHC and the Bank agree to indemnify and hold Sandler O’Neill and its affiliates and their respective partners, directors, officers, employees, agents and controlling persons within the meaning of Section 15 of the Securities Act of 1933 or Section 20 of the Securities Exchange Act of 1934 (Sandler O’Neill and each such person being an “Indemnified Party”) harmless from and against any and all losses, claims, damages and liabilities, joint or several, to which such Indemnified Party may become subject under applicable federal or state law, or otherwise, related to or arising out of the Reorganization or the Offerings or the engagement of Sandler O’Neill pursuant to, or the performance by Sandler O’Neill of the services contemplated by, this letter, and will reimburse any Indemnified Party for all expenses (including reasonable legal fees and expenses) as they are incurred, including expenses incurred in connection with the investigation of, preparation for or defense of any pending or threatened claim or any action or proceeding arising therefrom, whether or not such Indemnified Party is a party; provided, however, that the Bank, the MHC and the Holding Company will not be liable in any such case to the extent that any such loss, claim, damage, liability or expense (i) arises out of or is based upon any untrue statement of a material fact or the omission of a material fact required to be stated therein or necessary to make not misleading any statements contained in any final prospectus, or any amendment or supplement thereto, or any of the applications, notices, filings or documents related thereto made in reliance on and in conformity with written information furnished to the Bank, the MHC or the Holding Company by Sandler O’Neill expressly for use therein, or (ii) is primarily attributable to the gross negligence, willful misconduct or bad faith of Sandler O’Neill. If the foregoing indemnification is unavailable for any reason, the Bank, the MHC and the Holding Company agree to contribute to such losses, claims, damages, liabilities and expenses in the proportion that its financial interest in the Reorganization and the Offerings bears to that of Sandler O’Neill.

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 6

DEFINITIVE AGREEMENT

Sandler O’Neill and the Bank and the Holding Company agree that (a) except as set forth in clause (b), the foregoing represents the general intention of the Bank, the Holding Company and Sandler O’Neill with respect to the services to be provided by Sandler O’Neill in connection with the Offerings, which will serve as a basis for Sandler O’Neill commencing activities, and (b) the only legal and binding obligations of the Bank, the Holding Company and Sandler O’Neill with respect to the subject matter hereof shall be (1) the Bank’s and the Holding Company’s obligation to reimburse costs and expenses pursuant to the section captioned “Costs and Expenses,” (2) those set forth under the captions “Confidentiality” and “Indemnification,” and (3) as set forth in a duly negotiated and executed definitive Agency Agreement to be entered into prior to the commencement of the Subscription Offering relating to the services of Sandler O’Neill in connection with the Offerings. Such Agency Agreement shall be in form and content satisfactory to Sandler O’Neill, the Bank, the MHC and the Holding Company and their respective counsel and shall contain standard indemnification provisions mutually acceptable to the Bank, the MHC and the Holding Company and Sandler O’Neill and consistent herewith.

Sandler O’Neill’s execution of such Agency Agreement shall also be subject to (i) Sandler O’Neill’s satisfaction with its investigation of the Bank’s, the MHC’s and the Holding Company’s business, financial condition and results of operations, (ii) preparation of offering materials that are satisfactory to Sandler O’Neill and its counsel, (iii) compliance with all relevant legal and regulatory requirements to the reasonable satisfaction of Sandler O’Neill’s counsel, (iv) agreement that the price established by the independent appraiser is reasonable and (v) market conditions at the time of the proposed offering. Sandler O’Neill may terminate this agreement if such Agency Agreement is not entered into prior to June 30, 2004.

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 7

Please confirm that the foregoing correctly sets forth our agreement by signing and returning to Sandler O’Neill the duplicate copy of this letter enclosed herewith.

[Sandler O’Neill & Partner, L.P. Letterhead]

December 11, 2003

SI Bancorp, Inc.

803 Main Street

Willimantic, CT 06226

The Savings Institute

803 Main Street

Willimantic, CT 06226

Attention:            Mr. Rheo Brouillard

                            President and Chief Executive Officer

Dear Mr. Brouillard:

Sandler O’Neill & Partners, L.P. (“Sandler O’Neill”) is pleased to act as conversion agent to The Savings Institute (the “Bank”) and SI Bancorp, Inc. (the “Holding Company”) in connection with the proposed reorganization of the Bank and the Holding Company and the formation of a middle-tier stock holding company (the “Reorganization”). This letter is to confirm the terms and conditions of our engagement.

SERVICES AND FEES

In our role as Conversion Agent, we anticipate that our services will include the services outlined below, each as may be necessary and as the Bank and the Holding Company may reasonably request:

Each of these services is further described in Appendix A to this agreement.

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 2

For its services hereunder, the Bank and the Holding Company agree to pay Sandler O’Neill a fee of $25,000. The fee set forth above is based upon the requirements of current regulations and the Plan of Reorganization as currently contemplated. Any unusual or additional items or duplication of service required as a result of a material change in the regulations or the Plan of Reorganization, or a material delay or other similar events may result in extra charges which will be covered in a separate agreement if and when they occur.

All fees under this agreement shall be payable in cash, as follows: (a) $5,000 payable upon execution of this agreement by the Bank, which shall be non-refundable; and (b) the balance upon the completion of the Reorganization.

COSTS AND EXPENSES

In addition to any fees that may be payable to Sandler O’Neill hereunder, the Bank and the Holding Company agree to reimburse Sandler O’Neill, upon request made from time to time, for its reasonable out-of-pocket expenses incurred in connection with its engagement hereunder regardless of whether the Reorganization is consummated, including, without limitation, travel, lodging, food, telephone, postage, listings, forms and other similar expenses; provided, however, that Sandler O’Neill shall document such expenses to the reasonable satisfaction of the Bank and the Holding Company. The provisions of this paragraph are not intended to apply to or in any way impair the indemnification provisions of this agreement.

As is customary, all costs and expenses associated with the Conversion Center will be borne by the Bank and the Holding Company. In addition, all taxes however designated, arising from or based upon this agreement or the payments made to Sandler O’Neill pursuant hereto, including, but not limited to, any applicable sales, use, excise and similar taxes, shall be paid by the Bank and the Holding Company as the same become due, and the Bank and the Holding Company shall, upon request by Sandler O’Neill, pay the same either to Sandler O’Neill or to the appropriate taxing authority at any time during, or after the termination of, this Agreement; provided, however, that the Bank and the Holding Company shall not be responsible for the payment of any state, federal, or local franchise or income taxes based upon the net income of Sandler O’Neill.

RELIANCE ON INFORMATION PROVIDED

The Bank and the Holding Company will provide Sandler O’Neill with such information as Sandler O’Neill may reasonably require to carry out its duties. The Bank and the Holding

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 3

Company recognize and confirms that Sandler O’Neill (a) will use and rely on such information in performing the services contemplated by this agreement without having independently verified the same, and (b) does not assume responsibility for the accuracy or completeness of the information. The Bank and the Holding Company will also inform Sandler O’Neill within a reasonable period of time of any changes in the Plan of Reorganization which require changes in Sandler O’Neill’s services. If a substantial expense results from any such change, the parties shall negotiate an equitable adjustment in the fee.

LIMITATIONS

Sandler O’Neill, as Conversion Agent hereunder, (a) shall have no duties or obligations other than those specifically set forth herein; (b) will be regarded as making no representations and having no responsibilities as to the validity, sufficiency, value or genuineness of any order form or any stock certificates or the shares represented thereby, and will not be required to and will make no representations as to the validity, value or genuineness of the offer; (c) shall not be liable to any person, firm or corporation including the Bank and the Holding Company by reason of any error of judgment or for any act done by it in good faith, or for any mistake of law or fact in connection with this agreement and the performance hereof unless caused by or arising out of its own willful misconduct, bad faith or gross negligence; (d) will not be obliged to take any legal action hereunder which might in its judgment involve any expense or liability, unless it shall have been furnished with reasonable indemnity satisfactory to it; and (e) may rely on and shall be protected in acting in reliance upon any certificate, instrument, opinion, notice, letter, telex, telegram, or other document or security delivered to it and in good faith believed by it to be genuine and to have been signed by the proper party or parties.

Anything in this agreement to the contrary notwithstanding, in no event shall Sandler O’Neill be liable for special, indirect or consequential loss or damage of any kind whatsoever (including but not limited to lost profits), even if Sandler O’Neill has been advised of the likelihood of such loss or damage and regardless of form of action.

INDEMNIFICATION

The Bank and the Holding Company agree to indemnify and hold Sandler O’Neill and its affiliates and their respective partners, directors, officers, employees, agents and controlling persons (Sandler O’Neill and each such person being an “Indemnified Party”) harmless from and against any and all losses, claims, damages and liabilities, joint or several, to which such Indemnified Party may become subject under applicable federal or state law, or otherwise, related

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 4

to or arising out of the engagement of Sandler O’Neill pursuant to, and the performance by Sandler O’Neill of the services contemplated by this letter, and will reimburse any Indemnified Party for all expenses (including reasonable counsel fees and expenses) as they are incurred, including expenses incurred in connection with the investigation of, preparation for or defense of any pending or threatened claim or any action or proceeding arising therefrom, whether or not such Indemnified Party is a party. The Bank and the Holding Company will not be liable under the foregoing indemnification provision to the extent that any loss, claim, damage, liability or expense is found in a final judgment by a court of competent jurisdiction to have resulted primarily from Sandler O’Neill’s willful misconduct, bad faith or gross negligence.

MISCELLANEOUS

The following addresses shall be sufficient for written notices to each other:

The Agreement and appendix hereto constitute the entire Agreement between the parties with respect to the subject matter hereof and can be altered only by written consent signed by the parties. This Agreement is governed by the laws of the State of New York.

SI Bancorp, Inc.

The Savings Institute

December 11, 2003

Page 5

Please confirm that the foregoing correctly sets forth our agreement by signing and returning to Sandler O’Neill the duplicate copy of this letter enclosed herewith.

APPENDIX A

OUTLINE OF CONVERSION AGENT SERVICES

A - 1