[Harmony]

You'll have to start punishing him in ways that matter most. Whatever it is that he loves to do, you will have to let it be what he has tp give up whenever he is out of line.

He may be wanting the attention from her and instead of making it seem like he does, he chooses to fight with her and do anything else that will insure some type of reaction from her.

Try talking to her and let her know that she is older and should therefore be attempting to display that in her dealings with him. I know she's not a heck of a lot older, but she is old enough to understand that her meddling and picking on him will not be tolerated either.

Tell her to turn the tables on his "aggression." The next time that he is striking out at her, tell her to invite him into her room so that she can read a book to him or to watch TV with him.

Once she's done that enough times and has him hooked, he's going to want to do all that he can to continue spending that kind of time with her. You could even start complimenting him on how well he's been behaving and allow him to earn a special reward for "being a big boy."

He's frustrated that he is no longer the 'baby' in the family, but your daughter's role will never change. She was your FIRST and no one can knock her out of her spot, whereas the younger sister did just that to him.

Try it out and give it time to sink it. It should be a lot more favorable for you. Best wishes with this. :)