Bfs sister

So long story short my bf has always been okay with me having guy friends and I have this one we were bestfriend well my bs sister told his family I’m a whore and I was cheating on my bf w my friend and that I was werod for hanging out w a nerd so I cut my friend off and she just started dating him and texting him talking a lot of shit ab me idk what to do I can’t call her out bc her family goes off on anyone who offends their daughter