I banned my sisters girlfriend from our apartment.

My (21F) sister (18) was supposed to move to Maryland with my family this September for college but unfortunately didn’t receive enough Financial Aid so will be staying with my boyfriend & I in our city for a year. A few days ago my sister found out her girlfriend was cheating on her. There were very clear messages implicating her. There’s no doubt about it. This girlfriend has continued to manipulate and gaslight my sister. While what she did is extremely wrong, my sister is not innocent. When telling me about this, my sister admitted to getting violent with her girlfriend. Apparently when my sister confronted her girlfriend (I’ll call her girlfriend Z), Z wouldn’t let her leave. She attempted to restrain my sister and took her phone. This is abuse obviously. My sister has bruises on her arms from the restraint as well as a torn shirt. Now, my sister has also admitted to beating up Z. Multiple times. And this ain’t the first time she has put her hands on her either. My sisters excuse is that they’re both girls so it’s not as bad as a woman & a man being violent. That there isn’t any overpowering. I DON’T CARE. Domestic Abuse is UNACCEPTABLE. I won’t tolerate it & especially not in my apartment. I told my sister they are both wrong & that she needs to get help and go to therapy. I told her that she does not need to be in a relationship like this. Everyone is telling her this but she is believing Z’s lies that “it was never physical” (it’s very obvious that it was). Now, here’s where I may be in the wrong? I told my sister that Z is not allowed in our apartment. I won’t have any fighting in my house nor the police being called. I do not ever want to come home and have to break up a fight. I told her that if she can’t learn to keep her hands to herself then Z isn’t allowed over whatsoever. I said “i’m not gonna normalize that. it’s fucked up” My sister & Z aren’t together at this time but “are on speaking terms”. My sister replies with “girl what…” I texted my sister this morning to find that she blocked me. Also, my boyfriend & I will be the ones paying rent. My sister will be paying the electric bill. I don’t care that she’s upset with me. She should be upset with herself for being abusive. What other action should I take?

TL;DR – Ny sister & her girlfriend are abusive towards each other. It has escalated to physical violence. I don’t want this happening in my apartment.