Not having been a parent before you can read through this and just toss the advice if you don't think its practical...
But I would suggest having very set consequences for her not doing particular tasks, then immediately follow through if she defies you. For instance, if one of her chores is to wash the dishes, then you let her know ahead of time that if she chooses not to do the dishes, then that's okay, but she'll lose her TV priveleges for the next day or two. Or if she fights with a sibling, then she can't do X thing that she likes to do for some set period of time.
Like others have said, its a crazy time, and I think one of the best thing you could do is structure like crazy. So don't argue with her - just give her a task to do and a consequence if she doesn't do it. She can have her say, but if she does it in a nasty way, then she gets X consequence. And if she argues with that, then its another consequence. Then you follow through consistently, no exceptions. She's probably doing at least a little testing to see how far she can get, since she feels like she is growing up and wants to do more and more "older girl" things.
Anyhow, that's just my thought on it. Any way you look at it, its a tough time, so I wish you the best of luck!