GUYS/GIRLS HELP!!! :( TEENAGE ABUSIVE RELATIONSHIP?

My name is Angela and I have a younger sister that's sixteen (I'm 25....) I am currently engaged to my highschool sweetheart that I love dearly. My parents recently died and I am her current guardian, she is living with me and my boyfriend. I adore my sister, she's a beautiful young girl with a good heart--sadly she's submissive and very shy. Her attitude is very off these days, it's like she's afraid. When I confronted her about it, she locked the door and refused to talk to me. Yesterday, I came home from work early and found her on her bed with a boy, he was very rough towards her and I could see bruises on her body. I threw him out the house (I'm a black belt) and confronted her yet again. She refuses to answer me. I truly love her and I am trying to understand her relationship with her boyfriend. I have never been abused before and it surprised...I cannot say I understand because I don't. Please reply (I know it long), how should I approach her? Thank you. :)