I don't like my boyfriends younger sister?

She's extremely annoying. She wont quit talking in a freaking baby voice and she's 16 years old, plus she tries to compete with me on EVERYTHING shes extremely spoiled so she gets mad when she doesn't get her way and they just let her run about and do whatever just because theyre to scared to put in any rules down. Everytime I try to talk to my boyfriend about some of the things she does, since i'm not stupid and I was 16 once and i know all the cute little lies to tell parents, he yells at me said shes a little kid. She constantly competes with me for his attention and i'm trying my hardest to get my mouth shut because i know it's not my place to take him away from his sister, but should I tell him EXACTLY how I feel so he'll understand why i always get up and walk away every time she opens her mouth. Theres so much more that she does like goes way to far to try and be funny where she crosses the line, even for me, she always acts like a dumbass!!! and the stuff she says isn't funny or interesting its just annoying.