You're probably missing some facts.  He would not inherit from a stepmother, unless she left a will.  My guess is that the father had put it in joint tenancy with him, before his death, and he didn't know about it.  Ignorance is no defense if he cashed the check.  Get the deed.  If the stepmother's name was on it, use her date of death.  If not, use the father's date of death.