need a rant

Eve • 🤰🏼🌈👶🏼
So the last couple of weeks my boyfriends youngest sister (15) has been around a lot. Stayed over for a week straight and then stayed for the odd day here and there. Well on Friday, bfs brothers wife messaged me asking if I had a problem with her and I knew why. 
On Thursday, little sis uploaded a pic on fb of her, my bf and their cuz and some girl liked it. She asked me who said girl was and I didn't have a clue, my bf was friends with her so I told her to go ask him who she was (coz she wanted to know!) came back in and said that my bf had told her to tell me to stop being an insecure bitch? Huh! She's gone and told him I was kicking off! Wtf then went through this girls pics omg look she's a tramp etc. So she shit stirred between be and bf. Therefore, I knew she was the reason I had this text. Messaged her back saying no why. She replies wonder why you think I'm fake? The conversation I had with little sis, I said did. You notice how 'mary' called her nan? She never does! Maybe it's coz everyone else was there? Then little sis goes yeah and she was putting in an English accent! I said yeah when we went away 2 years ago she was talking in an Essex accent. Little sis hahahha she's so fake! Apparently I said it now? Then on Thursday, she said saolerhing about Mary and apparently I said fake when her name was mentioned? Now my bf was there and even he has said I didn't say fuck all! I've said all this to Mary but she doesn't believe me and believes little sis. I'm so upset by all this, Mary's even written a status about me and I've lit done nothing wrong! I haven't said anything but little sis was slagging Mary off earlier in the day before coming over to line and then calling her fake! 
Little sis also rang me up that night and started giving me abuse over the phone etc. 
Today my bf popped out and had a lift of a mutual friend of him and his bro. The friend 'bob' said I heard that you and brother have had a fight or a falling out? Wth! So brother has obvv said he ain't talking to my bf! 
My question, what has my bf done? Nothing! I know that his brother and wife don't like me very much as it is, I don't know why but whatever! But this time I've literally done nothing wrong!
Little sis slags Mary off to me all the time and says oh she said that about you etc! And with other people, slags them of and then licking their ass when she sees them! 
It's got to the point now where I won't go around his mums coz im gunna feel uncomfortable and cba for the dirty looks ima get and the whispers in the corner. I want to move away just because his little sis is a shit stirring bully!
My bf is furious with them both (little sis and Mary) and now his brother coz I've been so upset over it and he knows I've literally done nothing wrong! 
Coz his bro ain't talking to him now I'm scared. We've not long lost our baby but will be trying again when we can. What if this never gets sorted? His brother gunna be that petty that he won't be in our child's life? Or when we get married he won't be there? Maybe I'm over thinking but I'm actually worried about this. 
As for the little sis, I want nothing to do with her anymore. I'm done with all her bullshit she constantly puts her family through! There's so much I could say to my bf about her that she won't like but I don't want to put my bf through any more stress! 
Any advice for me ladies? I'm going out of my mind worrying about everything 😩 and if you read it all, thank you! 😘