Asher, to be quite honest they were serious. At least he was, but since he missed the girl probably didn't notice anything special. These two get really rough at each other sometimes, I can tell.
As for your question, I have had the same mixed emotions regarding these pictures. I, the father, was standing close when my son tried to whack his sister. What did I do? I took a picture. Am I getting used to seeing the evil in people by witnessing a normal and, hmm, healthy growing process of my kids and no longer feel the need to intervene? Or have I just grown a sixth sense and understood he's going to miss anyway?