How to let someone see the signs of abuse

My sister (18f) is dating a boyfriend for a few months. And they have already moved in together. She wants to go back home but he threatens to break up. In this short amount of time, he has called her ugly, fat, retarded. At least what told me. She has burned all her bridges with her friends and family. I am so worried. He has threatened to shoot her for cheating. He just says that. I shoot you if you cheat. And he goes through her phone randomly to find stuff. And will make issues going through her phone out of nothing. I keep telling her to RUN. This relationship is not good. It will lead you down a bad path. She says one more chance. What can I do to open her eyes. I think She doesn’t want to be alone. There’s more I can describe but theres so many red flags. She has to make the choice which is the hardest part of watching this