Not affiliated with the government
You can report Social Security fraud here :
Are you sure these were SSI payments? If your father died, you may have qualified for survivor benefits with your mother possibly (likely) your representative payee. Unlike SSI, folks can receive survivor benefits irrespective of their assets (ex. inheritances).
The following indicates a five-year statute of limitations for Social Security criminal activity, but it appears there is no time limit for seeking return of funds.