😤 His best friend is his sister in-law

A while ago she n me had an argument which ended up with her call me an immature, jealous person threatening to hurt me and said life was better before I was with my husband. I told my SO n he brushed it off saying she can be crazy. I just avoid her and get through family events to be respectful her n me don’t speaks n we don’t need to I get along perfectly with My brother in law. However she FaceTime him talks with him, messages him the other night at a family gather he sang Karaoke with her until 3am. I try not to react although it kill me on the inside. I know he isn’t off having sex with her but I just feel disrespected!! To top it all off he just txted me he is going to lunch with her! How should I react!! He obviously doesn’t listen to me I don’t want to be a jealous b$@& but I just can’t help it!!! Advice please!!!! 😪😪