Yes, there was an interesting article I read somewhere in entertainment news that followed the whole path of how his Oscar will eventually get to Matilda. The upshot is that it is basically held in trust for her until she is 18. Now, simply because his family accepted it doesn't mean they get to keep it till then. I think, from what I read, it is legally to be turned over to his closest kin, which if that kin is a minor, it's to be held by the minor child's parent or guardian, this being Michelle Williams. Michelle, as Matilda's only surviving parent, is to hold the Oscar in trust till she's of age. A whole bunch of paperwork was involved, just like any trust for a minor child would be. Then, upon her 18th birthday, before his Oscar is turned over to her, Matilda will have to sign off on all sorts of paperwork, basically agreeing to the condition that she will not sell the Oscar, and that if she does, her first offer is to sell it back to The Academy for $1.00.
It was a pretty interesting article - if I can find it, I'll link it.
EDIT: Found it -
Here's the whole article: