It is a crime to steal *anything* that does not belong to you, whether it has a monetary value or not. Your  mother can press charges against the cousin for theft--that would be her simplest, least costly option. The charges can very likely be resolved by the return of the ashes (subject to prosecutor and judge approval). Or she can file a lawsuit in chancery court (a part or division of county court--not, unfortunately, in small claims court) for a court order requiring him to return the ashes.