My boyfriend is divorced and is very much a part of his children's life. Their mother is the custodial parent but is inattentive at best. His 15 year old daughter sleeps in his bed during biweekly weekend visits. I think he needs to grow a set of balls and set boundaries and say "no". He is afraid of alienating her. He sets no boundaries as it is in any area...the two daughters control his life. But how is even wanting to sleep with your father as a teen and with your teen as a father remotely normal??? It creeps me out.

But hey if not then she just sleeps with him no wrong in that!!
If the biological mother of the girl is inattentive, maybe the girl feels some sort of stability being with her father, and hence comfort by sleeping in his bed. If they were taking a bath together (not a shower, as I know families that shower together) or you walked in on something, then I would be creeped out.
But to me it honestly sounds like he's some sort of stability in her life. Have you offered for her to sleep with you? Maybe by giving her another adult/female view on things can break her from being so dependent on her father. But do it tactfully and gracefully. Be her friend first and then start to ask the questions or hint.
Or she could hate you, and knows thats by sleeping in his bed, you're not there taking her father away from her.
Girls are a mystery.
She only sees him once in a while, and he doesn't set boundaries for them. While their mother is "inattentive" and from the sound of it not a very good parental figure, he's there FOR them. Some boundaries may be good, but at least he connects with them(letting them "control" his life). So of course they feel good with him. How old is his other daughter? I actually wonder why only the 15-year old sleeps in his bed.
I think if my parents had been divorced, i would also have spent as much time as possible with my dad, including snuggling up in the evening and possibly falling asleep while talking...as long as there's no other indications that suggest something more, i doubt there's any reason to be alarmed.
I'm a guy with 35 and I live alone with my 14yr boy and still sleep with.
I posted my concerns just a wile ago.
Things went a litle to far now and I feel caught in a situacion I don't know how to solve.
If you like your boyfriend try to do something. I already lost 2 girlfriends because of my singular situacion. (sorry 4 my bad english)
Not normal. No matter the situation, icky or not, boundaries are critical for kids.