Oh, so what you ment is that it wasn't a inheritance from Lynette at all, but rather from either her mom or dad aka one of the MC's grandparents dying ?
If that's what you ment, then there still would be no legal way to confuse who actually died. The MC would have been notified of the death (if he was determined to be one of the benefactors) and then, being of legal age, he has to sign and accept his inheritance and no he can't pawn that of to his dad.
And given what he got, it would have to have been via a will, because as a grandson you don't inherit your mothers clothes or diary, but you do climb the inheritance ladder if there are no children, if what you say is true and Lynette was an only child it would have been the remaining spouse(grandparent) and then the MC.
So the inheritance via will would absolutely be possible, but not sneak it past the MC.