She is 18, which my BF uses as an excuse for why it's less strange, saying she feels like a little sister to him. She tries to engage me far more than my BF does when we're together. I know she can tell the whole situation is weird and my BF has told her I feel like I don't belong. She is very close with him though and has no problem getting up in his space, taking pics with him (one of her lockscreen photos is of the two of them), and confiding in him/asking him to hang out. I'm sure she has some feelings for him, I think it would be impossible not to considering their dynamic and my BF has stated that himself - but I don't think she would make a move.