Rivalry?

Even when he is at his best, most gentle, most adoring and kind, Jasper is a menace to Josie. He squeezes her like he is deflating an air mattress. He gives her hickeys on her forehead. He picks her up by the waist, shakes her arms, and grinds his chin into her head. He pokes at her eyes and has been known to bite her toes. And these are his expressions of affection. Only a couple of times so far has he expressed true annoyance with her, and never (so far) anger. On a few occasions when he has awoken from a nap grumpy and wanted to be held, only to discover a nursing mother, he has demanded that I "get rid of her." And the other morning when she finally mobilized her arms enough to swing around a hand and grab a fist full of his hair, he burst into tears and cried, "She HURT me!" Josie has never responded so dramatically. We try to meet each painful encounter with a clear definition of what might hurt her, and examples of what he can do with his new sister (not much yet, I am afraid). But it is tempting to just say that she is off limits until she turns one. I want them to be comrades, co-conspirators, friends. Is it inevitable that they will always be somewhat at odds for attention and other familial resources? What role do "nature" and "nurture" play in sibling relationships? Sometimes I wonder if the solution is eventually providing them a common enemy: us.