Whenever he is behaving inappropriately, get him to do it "the right way." For instance, if he is lying on her, ask him to sit up. Once he does that, give him TONS of positive attention, saying something like "Oh look! Bobby is sitting SO nicely beside his baby sister! That is SO nice! You are a SWEETIE-PIE! Bobby why don't you go to the kitchen and get one of those new cookies Mommy bought you and bring them here to eat?" These interventions constitute the CLeaR Method - Comment, Label, Reward. The structure is a powerful reinforcer and can really increase the chances of appropriate behavior occurring in the future. I'd suggest you use it every time your little guy is engaging in inappropriate behavior around the baby. After a few times of giving a reward, however, start to thin it out (see Raise Your Kids without Raising Your Voice for complete instructions).

Sarah Chana