About the behaviour towards sister, girl at water play area, etc etc... Have you looked at his diet?
DS was getting more and more violent, when finally I had an "aha" moment after he lost it 20 minutes after having a DumDum lollipop. Over the rest of the year we worked it out and realized he can't have anything based on corn syrup. Colors might be a problem as well, but it's hard to find things with artificial colors that don't have corn syrup in it, and he's not willing to experiment. However I do remember one WEIRD Easter-time with him licking his fingers while dying eggs (he loved the vinegar) and getting into a really freaky mood.
Once we got rid of those things from his diet, he became MUCH more calm. Before that, I had to physically separate him from our cat, because I couldn't get him to be *at all* gentle with her. No you can't expect much from a 2 year old with a cat, but I had plenty of friends who had cats when their kids were that age and they pretty quickly learn that petting didn't mean pressing down so hard that the cat's belly touched the floor...he never got that. Sadly she got tremendously sick, then while hubby was cleaning up after her with the door open, she ran away... So we never got to see if it was causing the weirdness with how he treated the cat, too, and I'd realized how very allergic I am to indoor cats, so we aren't getting another.
Anyway, just an idea based on our experience!