C.W.
I hated that I was becoming the "yelling mom" and was looking for real solutions for my 4 year old at the end of last school year. "Real" solutions being something other than what my grandmother was telling me... to hit. I do not hit my kids, it is a conscious choice, so her solution was not an option and not something that would work at all anyway. The Faber and Mazlish books are great for getting a handle on what is actually going on in those kids' heads. Please please please read them.
Other than that, you can try positive reinforcement with them. This will work for all of them, even with the age difference. We currently use a system that we call "good boy bucks". My husband created some "good boy bucks" with Microsoft Publisher. It looks like a small dollar bill with my son's face in the middle. We made 1, 2, and 5 dollar increments. Whenever he is doing something good - playing nicely with his sister, being quiet at a time when he needs to be, getting ready without being asked, cleaning up without being asked... anything GOOD - he gets a buck. You decide how many bucks based on the behavior. If it is really good, he gets more bucks. He keeps them in a "wallet" on the refrigerator next to a menu of items he can buy with his bucks. Example: 2 bucks = watching one episode of a TV show, 4 bucks = long show or movie, 6 bucks = ice cream at local ice cream shop, 15 bucks = mini golfing, 45 bucks = this Playmobil playset that we have been sitting on for a while. He can use or save his bucks as he chooses, so far he has been saving for the Playmobil set. Just as he earns bucks for good behavior, he loses them for poor behavior (this is becoming less frequent and we have only used this for about 2-3 weeks). This has really changed things in our household in the past few weeks.
I'm sure that it is hard going back and forth between houses and getting to the bottom of their feelings is going to be a big first step for you, but after that, using some positive reinforcement is really going to keep them moving in a good direction.
Best wishes.