Baby momma trying to ruin us

Okay so my boyfriend has two daughters one is 10 and 7 and when the smallest one was 3 months she left. She was a drug addict and when to jail for fraud. Grand theft. Identity theft and drugs . And she got out and went to rehab. But this crazy bitch is out and I mean she's crazy and she's trying to part of their lives but she really only trying to get him back and they recently talked on the phone and he said she asked him to get her pills and drugs like wth she's not even better and I feel like she shouldn't have any contact but I have no say because they kids aren't mine he swears he'll never hurt me especially leaving me for her but I'm scared not because he'll leave me but like the situation will be too much for me I mean she's crazy and she already threatened me and she found my Facebook and it's not saying anything bad but they had already talked and I really don't know what to say or do