Sexual tension of an adolescent, who maybe realized on some level, his lack of potential sexual partners, and assumedly a virgin. The sexual relationship between her and Will, which he was listening to, and likely turned on by. And, the dynamics of that in a house where there was an existing power structure, who the real "man of the house was". I agree those screws could have been turned much tighter, though it is already a really uncomfortable watch.
Imagine being a 17 year old virgin, who may never have sex, and have to live with his maniac father for the rest of his life. I think things ended ok in the end for everyone. Sometimes, opting out is better.
I do think there was some strictly physical attraction from her too.