I think what is coming across is that you are trying to be fair in your reactions, and really trying to give us a realistic view and not just paint your sister as a bad person.
Like others said, it's hard as a parent when we are helpless to help our own kids, and the family that we're supposed to be able to depend on seems to be working against us.
I have someone in my family(A) whose cousin(B) wanted to move out of her home (for whatever reasons). B was over 21 and asked if she could move in with A. The parents of B went ballistic and cut A off as well as A's parents. They wanted their daughter to have no support so she would move back with them. Well, B 'grew up' in whatever way she wanted, and eventually got her own place. She never did move back with her parents. Years later, she made up with her parents, but the relationship between the two sisters (A & B's mothers) has remained strained. Of course, past history had some influence whether or not the sisters used this event to not talk to each other any longer.
I don't know. I see these types of relationship struggles, and think we have to decide, ultimately what we want.
What kind of relationship do you want to have with your sister, and how much are you willing to "sacrifice" for the peace of the family?
This story is very strange....the "child" was 21 yo!