Boyfriend's "like a sister" (long sorry also wrong topic lol)

Ok so to start off I am 15 and my bf is 16, we have been together for about a year and a half now and we both love each other very much and have been through so much. But he has a friend that's a girl and they have been friends since like kindergarten and they live barley a mile apart and she is always over at his house. BUT her mom and his are good friends and when she is over there she just hangs out with his parents/her room ( yes she has an actual room over there) because she doesn't like staying at her house alone since her mom is always working. My bf's mom has talked to me about it and has said she knows it must be upsetting to me but has assured me many times she would personally beat the shit out of him if they where to ever do something. And also I like to slap him on the but ever so often ( what can I say lol) and one time he bent over at his house and both me and "the girl" where walking behind him and I trapped that ass and he jumped straight up and said which one of you was that very defensively and I said it was me and asked why he said it like that and he said that if that was "the girl" he was gonna to say what the fuck and tell her to go home, which really made me feel better. We all go to the same school and me and her have all the same classes and are good friends and my bf doesn't have any of ours. I am still pretty jealous of her tho, like I don't think they are doing anything together they both literally seem annoyed with the other's general existence, but still it is kinda unnerving. how would y'all feel?