I need advice

How would I get over the fact that my bf sees a woman that he's fucked in the past? She is with his brother now so they see each other often. I can't get over the fact that they still see and talk to each other bc if it were me he would be like hell no and tell me to stop talking to that person. I don't want this to eat me alive. I just get this bad vibe from her and I just don't like the situation. Oh did I forget to mention she is married to another person and has a kid. To me she is a snake and nothing more. I just had to get this off my chest. Can anyone help me or do I just need to forget about her focus on my relationship