Drama! at its best.

So I'm 17 turning 18 in a few days (7) and I'm 11 weeks pregnant & my fiancés sister seems to keep talking about me behind my back and my fiancé and his sister keep having silly little fights over me" my fiancé told his sister to not talk to me and that I wanted nothing to do with her which I said none of that and she really thinks I would say something like that and I don't know how to defuse thi situation honestly I have no idea what to do... advice would be great!