Two things leap out from your question. One is that you have been badly treated by this person in the past, repeatedly. The other is that you still care for her, and feel responsibilty towards her. It is great that you are a caring person and you should feel proud of that aspect of your character.
But...sister or not, this is now another adult who has made their decisions about how their life is. I can't see that you can do much about that - distance or none. I agree that it sounds as if she has landed herself a control frwak. But that's the essence of the case - she chooses to be with him. She isn't a minor, and even if easily swayed I assume she is not classifiable as emotionally / mentally vulnerable or at risk.
Granted this last might change - but it can for any one of us as we go through life.
I think that you are doing enough by being there for her, now or at a future point, and in the meantime focusing more on your own life and needs.