Sibling rivalry already?

I hate to say that sibling rivalry has begun rather suddenly. Within the space of a few days Ben has gone from playing roughly with Oliver to being really maliciously rough with Oliver. For example, he likes to push Oliver's head to the floor when he is sitting and then sit on his head and he likes to hit and kick and jump on him. Oma Donna thought it might be because Ben knew I took Oliver to California with me and he is jealous. We'll have to figure out how to deal with it. I've yelled a couple of times when Ben appeared to be about to suffocate Oliver and he quickly picked up that habit and has yelled "No Livvy!" at the top of his lungs a few times. So I feel like that tactic has done nothing except give Ben another bad habit. We will try some time outs.