Powerless & frustrated

Has anyone else had to sit & watch while someone you care about stays in an abusive relationship and there’s nothing you can do about it? It’s so frustrating. My little sis has been dating this douchebag for almost a year now. She constantly complains about him and says she’s “done”, but within hours she’s caving to his bullshit again. Earlier tonight she was telling me how he made her feel so bad today and she was crying. And then next thing I know she has already forgiven him 🙄 Not a single person in our family approves, her therapist has told her it’s an extremely toxic relationship, she’s so unhappy and yet, for some reason, she stays. I just want it to end. I hate seeing her choose to make herself so miserable and I just can’t comprehend why.