My thoughts on the incest scene
In most of these movies the house makes the people do weird things. Remember in Amityville Dollhouse when the lady wanted to "do it" her husband's older son because the Dollhouse was putting those thoughts in her head? I think maybe it was like that in this movie too. Because maybe the house influenced this relationship knowing it was wrong and taboo in America and also because they're teenagers and yadda yadda. You know what I'm saying?
I don't necessarily think she wanted it, I just think the house influenced it from the very beginning when they moved in and him becoming possessed just took it the extra notch it needed to go "too far" and when she says she doesn't feel guilty, you know she really was feeling guilty, she was just being clingy and hoping that he wasn't regretting it so she wouldn't feel like even more of a tramp and wouldn't wallow even more in self-pity for what she had done.
I don't know, maybe I've been in too many psych classes to be analyzing a fictional incestuous relationship. :/ But that's just my take on it, from a psychological point of view.
You would think killing people would make them like you but it doesn't. It just makes them dead.