boyfriend's little sister

so this little girl is so immature let's start there. she's aggravating and she doesn't like me so therefore when I saw her and her mom at the mall (I was with my mom) and we all started greetings and talking and then her mom left and then mine she gave me a stank look and I rolled my eyes and left to go with my mom. I mean I should've smiled bc I think that'll piss her off more but anyways my boyfriend told me that she doesn't like me bc of who she is as a person.

um que?

and so I told him that i don't like her bc she doesn't like me, I mean I try to tolerate her but she's so disrespectful. then I told my boyfriend another time she was being aggravating he said "i just want you two to get along I don't want you two to have beef with each other and don't talk about my sister like that, yeah I think so too but still" and I said "then tell her not to have an attitude with me so we can get along bc I didn't do shit to her and how is it okay for you to tell me my mom's a bitch but I can't call your sister that?" (my mom's a bitch he stated a fact so it didn't hurt me but that's my mom and he complains that I called his sister a bitch even though we both know it's true yeah k that makes sense)

and he refuses to tell her bc he's scared of her but I'm not lmao but anyways can ya'll please provide advice and tell me if I'm the one who's at fault bc I didn't do anything to her for her to not like me (she's like 2 years younger than me).