I don’t know what to do or say.

My sister is in an abusive relationship with a guy, he’s hit her a few times and the police have been involved numerous times. Twice now we’ve had to move her out of his house. She’s lived with us for close to a year now, and has been dating other people, but this week informed us that she is moving out and is moving back in with him “because it’s closer to her work”

I am FUCKING pissed. I don’t know what to do, but at this moment I want absolutely nothing to do with her. I don’t want my kids around her. My kids know that this man has hit her (okay maybe not hit, but know that he was really mean to her and made her feel scared enough that she had to leave), how do I explain to them that she went back to him? I obviously can’t make her choices for her, but I don’t want to help her next time.