My sister's in an abusive relationship

My sister is in an abusive relationship. I always knew there was something off about she and her bf. He has broken her phone multiple times (phones he did not buy) and i have noticed bruises on her legs etc. Tonight, after some argument, he hit her outside our house with a bottle of perfume and it broke on her back. She finally admit that he hit her tonight but she won't admit that he's been like this all along. She has been with him for about 5 years now and im afraid that even after tonight's incident she won't leave him alone. She's not happy and everyone knows it but i don't know why she's still with him. What can i do to break through to her?? I honestly think she will still be with him after tonight and im scared for her safety.