I respectfully disagree with you.
You could've auctioned the domains on his behalf (with the permission of the family) and kept a margin for your efforts and gave the rest to the family.
Even if you (for example) take 50% and give 50% back to the family, I'd say that's the right thing.
It's not abut 5 years or 8 years, or 20, it's about value, conscience & trust. I don't think many people know the value of the domains and thus they'd feel it to be a burden.
Anyways, there's a lot that can be said in this matter. May be you've tried to convince about things like auctions, may be you didn't. As I don't know the whole story, I'd rather not comment about it further than what I've already said.