M.M.
I've read most everyone else's responses already so ditto to what everyone has said. I have a 2 year old son and 1 year old daughter. My son has been displaying the same behavior towards his sister but has gotten better over time. We have modeled how to share or trade toys when she has taken something from him; we used time outs for the extreme situations of physical reactions like hitting or pushing. Right now my son is very sensitive to people's emotions so we've identified how it makes his sister sad (it helps that she cries) to make the point to share. But we also acknowledge that it can be hard to share, "especially when sister doesn't ask because she can't yet and she's still learning so we have to help teach her how to play, too." He seems to calm down more quickly when we first acknowledge what he might be feeling and then go into the whole sharing/trading/learning to play together thing. When I've been occupied and can't play with them, I have physically separated them with a gate or something so that he gets some undisturbed time for a bit. As an older sib myself, I understand my son's desire to be left alone. Don't we all sometimes just want our own space? :-)