Plz help me! my boyfriends sis always start drama!?

i love my bf and he went to bc two months ago and b4 he left he gave me his fb password. so his sis hates me wit a passion idk y she tlks bad @ me to the family n mkes up stories @ me like when i went to her house n left der was a ball of **** in da toilet n i pour eyeliner everywhere. i log on his fb to see shes already log on and shes been messaging this guy @ me idk who he is but he says i ask guys for kisses n bend over so i block him so she wont start problems. next thing u know i go on my bfs facebook n see a post wit all this profanity " u bit++ block him cuz u know its true im not stupid" idk what to do i feel like crying sometimes i never done nothing to her n i never cheat on my bf