Relationship help

Sorry if this is long. I've been with my boyfriend for six years. We have an age difference im 24 he's 42 his daughters 17. She doesn't really like that were together which i get. She blatantly disrespectful and mean to me but i ignore it. We dont live together but im there 3 or 4 nights a week i work nightshift so im at work 3 nights a week. Last year he admitted his daughter to rehab for an eating disorder she seemed to be doing well. In February her boyfriend of a year got caught cheating. Last night i found a razor in the bathroom disassembled and in a box by the tampons. I obviously told my boyfriend and i assumed he talked to her this morning. Things have been hard for us with her not approving the relationship. We've been arguing the past few weeks and tonight he said to me you'll always come second. Being a nurse alot of her behavior is attention seeking i see it and i know it well but i of course urge him to get her help and dont butt in where im not wanted. Im concerned that shes going to tell them that our relationship is making her depressed because everyone knows he'd say okay and we'd be done. I just dont know what to do. Any advice on any part of the situation would be helpful