J.P.
Maybe your daughter could ask for a hug? Not just go give one? My other thought is what is your daughter like to your son when you aren't around? Please don't take this the wrong way, but is it at all possible that she is sweet to him when you are around and lets her jealousy of him show when you aren't? It is the hitting himself when he hears or sees her that makes me think this. I am not a behaviorist at all, and he could just be doing it because he doesn't want the hugs and kisses, but it was the first thing that popped into my head. Good luck.