My sister won't leave an abusive relationship?

My sister is 23 years old and her boyfirend is 26. She sits around and just lets him hit her in the face and slam her against the wall. Me, my parents and her friend have all tried to tell her to stay away from him and leave, but she's too stupid to listen. One time her friend called my parents saying that my sister and her boyfriend got into a fight, because she wanted to go out with her friends and he kept blocking the door, and he choked her, punched her and slammed her against the wall. He's a controlling maniac. When he came over our house for memorial day, my dad tried to kick his a**, but my mom and I had to hold him back. My sister is so stupid, and she deserves better. I'm afraid that he may try to kill her or something. I'm just so tired of this. I'm 20 years old, and not only do I stress about school, but I sit here and stress about my sister's safety. I'm just tired of it. Why does she keep letting him do this to her? What can we do to make her leave? I want to kill this guy!