If we're making book recommendations, let me throw Little Girls Can Be Mean into the hat. It's similar to Rosalind Wiseman, but really aimed at younger girls, when they're not yet into the social aggression over boys. One of the things I like about it is that it has at least one example of a girl being outrageous, and then a couple of chapters later, it has another example of bad behavior. Then you learn that it was the exact same incident, seen from both sides. It's a good reminder that no matter how horribly they behave, they're doing it for a reason, and the reason is usually understandable if you think about it. And sometimes you only see one side and get a warped view of the whole problem.
Which is not to say that I think that your niece isn't being a brat. But I can certainly understand the desire to help her, too - she is family, after all.
