Should I forgive this girl for causing my brother's death?

Okay almost two years ago, one of my best friend's was dating my brother but she had this other boyfriend who she had been with long before him. The other guy found out about them and told her if she didn't stop seeing him, he will kill him and her both so a week later she broke up with him and started dating my brother exclusively then the guy shot and killed him outside of our old house (we moved right after it happened) Me and our whole family were really mad at her since we (especially my parents) thought it was all her fault for cheating on the guy with my brother in the first place and I haven't really talked to her since and hardly even seen her. But now I work with her at the same place and see her all of the time and I want to be friends with her again but a part of me can't help but feel that if it wasn't for her my brother would still be here. Plus my mom wouldn't like it if we were friends again, since she still really hates her. What should I do?