"WhIcH oNe Is WrOnG?" Your sick ass son. I know your biased because you didn't give birth to him but you need to reasses the situation and look at the bigger picture. If he's being inappropriate with HIS OWN GODDAMN SISTER to the point where she felt the only rebuttal to him invading her privacy and making her feel uncomfortable was to push him down the stairs. What is he doing to other girls when your not watching and what has he done to her prior to this altercation? The severity of her response implies that this is nowhere near the first time he's done something like this. Make him understand that sexually harassing his sister or anyone else for that matter is WRONG and apologize to her for everything she has dealt with up to that point that you nor your partner had caught onto until now.

I have to wonder if this is the extent of what is going on, or if there's some type of sexual aggression going on here. The son may need treatment to keep him from becoming an adult sex offender.

If you discipline your daughter, you're going to teach her that she can't fight back when someone pervs on her, and yes, it WILL happen again--men use being pervs as weapons to terrorize women 'for fun', and yes, it happens to ALL WOMEN. You definitely need to figure out what's wrong with your stepson, though, because that's wildly inappropriate and developmentally strange as hell.