[Ennui.]

I’m sorry you went through that growing up. It’s scary to see it being repeated with your sister.
I don’t think there’s any way to force your sister to stop dating this guy. She has to come to the decision herself. It might take something happening for her to realize that this isn’t a healthy relationship that she wants out of.
You and your family can have a family meeting with her to talk to her about how you are feeling and why you are concerned. Let her know you’re worried about her health and safety because you’ve seen this before with your father. Tell her you care about her a lot, love her, and just want the best for her. But also let her know that you’re safe to come to if she is ever having a hard time and needs to talk to anyone.
Don’t yell at her. If it turns into an argument it’s better to walk away instead of allowing things to escalate. You can continue the conversation when things are calm.

I hope all goes well!