Boyfriend and his sister

Okay so my brother and I are close. So I understand my boyfriend and his sister being close and all but everyone thinks I'm just overreacting when I say they have a weird relationship. Like for example. He's 23 and she's 25 and she trims his toenails for him because if she doesn't get to do it she throws a fit and threatens to hurt herself. And he won't let ANYBODY but her do it for that reason. I think a grown man should be able to trim his own toenails. Or this past week he had an ingrown hair on his face and it was infected and swollen and hurting really bad and instead of letting me help him with it he suffered all week til his sister came home on the weekend to help him with it so he wouldn't upset her. Then she ended up putting something on his face that caused his skin to burn and he got mad that I said I wouldn't have put a chemical on his face and that she should have been smart enough not to do it. We just had a baby and she tries to act like the mom when we go places as a family. She always wants to carry the baby and the diaper bag and stand right next to him and he tries to come back and sit by me and she will freak out. She got mad that he bought me nice things for Christmas. She's mad that he lives with me. She gets jealous if we wanna do something alone. Is it just me or does it seem like his sister has something weird going on? It's hurting me now because I'm never allowed to do anything for him and I feel worthless... Any advice? Also, she's in law school so her and her whole family thinks she knows everything about everything so I think that's why he's convinced she's the only one smart enough to do anything for him.