We have a unique situation -- we live in the same house as my SIL, BIL, and nephew - 2 yrs 10 mo old, AP all the way, very verbal and mature. DH and I have DD, 10 mo old.
Our nephew is having trouble sharing, which doesn't concern me much--we negotiate and when necessary use social interaction consequences: "if you don't choose to share your toys we will have to leave now. we can't play together unless we are ready to share..." He catches on very fast and he'd rather play than hoard his toys.
The bigger concern is that he is often purposefully mean to her. He will put his feet in her face when he thinks nobody is looking or push her hands off the furniture so she must sit down or risk falling. He will limit her interaction with the toys and he has even pushed her over on purpose. This is only about 5% of the time. The rest of the time he interacts very well with her and is aware of her space and her needs. His mom is on top of his problem behaviors when she is present, but because of the house situation, that is not always the case. I'm on him as well, but it troubles me that the behaviors continue and he's getting sneakier about it as he realizes we won't tolerate it. At this point we verbally address the problem behavior and tell him it is not acceptable and why. He generally stops at that point. I guess what worries me is that I feel like I can't trust him. But then I remember that he is not even three and I can't expect him to behave at a level above his maturity.