You're 100% right, but the person would be informed about his/her pending inheritance and the guardian/trustee of the inheritance would have to act in the best interests of the beneficiary of the inheritance, or in other words throwing it into a box and storing it in your cellar would be a crime.
There are exactly two possibilities, someone gave Neil the box or Neil had it all along. And given that the MC was 3 weeks shy of turning 19, it's highly unlikely that anyone actually died.
Someone wanted the MC to have that box and we can only hope it was someone other than just DPC for story purposes.