Help please

I need opinions and I don’t want judgment but please tell me if you think I’m being a total bitch. My sister is in an abusive relationship and every time we think she’s out so goes right back in. The guys who has been hurting her is actively threaten me and my boyfriend. He said he would kill us he said he would get my son taken from me. Not that I believe he can do that. He’s posting videos of himself with guns and threatening all of my sisters family. I know she has to decide to get away from him and it’s her choice but if she chooses to stay will I be a bad sister or person if I tell her she can’t be in my life or my sons life anymore? I love her so much and I would do anything for her but I don’t want to fear for my safety and my family’s safety.

Edit: I am in contact with a police officer who’s building a case. I email him every thing. She was away from him. She doesn’t want to stay away. All the resources she’s been giving and she doesn’t use them. She had a place at a domestic violence house and she refused to go. She’s skipped out on all her counseling and refused to go stay with family out of state. I want to stand by her and eventually her boyfriend will get caught up. But she actively chooses to go back to him because she wants too. She doesn’t want help. I know what you guys are saying and I’ve been trying for over a year but every time I get her out and let her stay with me to keep her safe she does stuff like take my car in the middle of the night to get him. Go out of her way to get back in contact with him. And every time his threats towards me get more and more violent. I would not think this way if I did not try everything else first. This is the absolute last resort.