Love her. Love her. Continue doing what you are doing. Be there for her. Listen to her feelings without judgment. Reassure her. This is not the time to suggest good riddance to bad company. That will only make her defend him. She is likely too upset right now to listen to reason. Just keep telling her the truth: This is not her fault. Her boyfriend’s family blames her because they need someone to blame. However much she loved him, she couldn’t heal him. She did what she could. She actually did more than she should. He was an unstable young man. He tried to make her responsible for his life and ultimately did an extremely hostile and hurtful thing by setting her up to think she is responsible for his death. This is not an act of love. It’s an act of control.
If your sister continues to blame herself, it might be helpful to take her to a counselor. It may be too hard for her to tell you all about the relationship. She already knows that you and your folks disapproved of him. If he did assault her, she may not want to talk about that with you. She needs a place where she can share the full range of her feelings (anger, hurt, grief, betrayal and, yes, even love) without feeling that she might risk further disapproval.
The fact that you wrote to us tells me how much you care. Your sister undoubtedly knows you are there for her. Stay involved. Check in with her regularly. She is very young and needs your protection and care as she works through this. The love and support of family and friends can make a major difference in getting someone through a tragedy.
I wish you all well.