I have to say, you are blind.
You feel threatened by a 15 year old girl, but don't see the part her father plays in all of this?
She's rubbing against him, does he stop her? Since you said they often fall asleep in each others arms, I'm guessing he lets this happen. I'd be more worried about him then her.
So you want to marry a man that lets his child fondle him? I wonder why she's jealous of you. Many sexually abused children end up feeling like they're in love with their abuser, especially when it's a parent. This relationship between father and daughter is disturbing.
I'm not saying that he's sexually abusing his child, but he's obviously not doing anything to stop her sexual advances towards him. I have to wonder why. You should be wondering too.
Stop feeling threatened by her and find out what's going on to make her behave this way. This is a huge red flag! I wouldn't accept a ring from this man until I found out what he's been up to with his child.