If the statute of limitations has not tolled (expired), you can sue the family member for conversion, which is theft in a civil case to recover the funds.Another alternative is sue the family member and get a constructive trust as the remedy.  A constructive trust requires the individual to return the funds.  If the funds have been spent, items purchased with those funds or the value of those items are to be returned to your father's account/estate and the constructive trust can be used to trace the funds.