We're having this issue with DS1 (almost 4) and DS2 (13 months). It has only developed recently, like within the past couple of months. Ironically, for at least a year, he never touched his brother anything more than gently. But, DS1 has learned that doing something to DS2 gets a rise out of me, and is testing a lot more than before. Yesterday he told me that when the sitter was there, he likes to make DS1 sad. I asked him how, he said pinching his cheeks. I asked him how he though it made his brother feel, he said, "sad." I asked how it made him feel, he said, "happy." I told him that made me sad. Then, later, I said, "do you know what would make me happy? If you were gentle with DS2 all day tomorrow." He said he would try, so we'll see how it goes.
I'm kindof at a loss for this too. He also constantly snatches toys out of DS2's hands, tries take anything DS2 is playing with. I really wish I could find/think of a way to foster and reinforce more positive behavior. But, they get into some automatic behaviors, and I think this is unfortunately becoming one. When I get home (work 30 hours), DS1 is a really big challenge, and DS2 really just needs some loving.
I hope more people chime in on this one!!