A non legal sibling also doesn't have any standing.
Yes this can be done, even by a grandchild. It will take a lot of paperwork though. It will help more if your dad has any siblings willing to help but you as his heir puts you in line as well. In all states after adoption finalization, the legal parents are the adoptive parents. The state will likely make you prove you're related to her: 1. Get a copy of her amended birth certificate (the original is likely sealed anyway) which has her adoptive parents on it. 2. Get a copy of the current death certificate. 3. Get a copy of dad's birth certificate. 4. Get a copy of your birth certificate. All the birth certificates will prove you're related to her because your dad's will prove he's her son, yours will prove you're his son or daughter which shows you're an heir of hers. Hers will be to be prove the adoptive parents names.
A grandchild has zero legal standing. Forget it.
What is the point, who is going to see it in the future. ? It matters to you now but in 10 years time?
There is no point at all.