All I can say is that it is feasible that one family member is acting on behalf of several others who are entitled to receive benefits. Representative payees, other than parents, are required to open separate accounts for each beneficiary. The account should otherwise reflect the fact that the funds are held in a fiduciary capacity.
My first step would be to look at 2 - 3 months worth of activity in the account and see if it appeared the funds were being used for the benefit of others. My second would be to call the account owner and ask for an explanation. SSA's Guide for Representative Payees