DS1 is 3. He was 20 months when I had twins (they're 18mths now) and when they're all together he would deliberately pinch, push and thump them. It was driving everyone crackers - I couldn't leave him unattended for even a minute.
So, I've had a change of tack. When he hurts them, I tell him "if you hurt the twins, they get a treat and a cuddle." And then ignore him, and cuddle the twins and give them a treat (sticker, choc button etc.) The first couple of times we did this he went absolutely BALLISTIC, massive tantrum, lying on the floor drumming his heels etc. But now, he's stopped! He was annoyed with me for not letting him play with the mop bucket (why?!) and went to hit one of the babies and then stopped and said "no hit the babies. No treats."
He's pretty pissed off with this change of rules, but it's working! Just thought I'd share!