Mass. Gen. Laws ch. 32, § 17

Any option, election or right existing in any member may be exercised or enforced, if such member is incompetent or for any other reason is unable to exercise or enforce the same himself, by the spouse of such member if they are living together at the time any such action is instituted, or if there is no such spouse then by his guardian or conservator, or if there is no such spouse, guardian or conservator then by any other person found by the board to be acting in behalf of and for the best interests of such member.