How can I stop my 8 year old daughters attitude?

She's the oldest of 4 kids, but at 8 she acts as if she were 16. I'm not sure if this is "normal" but it's driving me nuts and I don't know how to stop it. She keeps her bedroom a huge mess, cries when she has to clean it, gets really mad when she doesn't get her way with her friends, and acts as if she's the Mommy to the other kids. I caught her my 2 year old today and she didn't see why she shouldn't be allowed to (even though I do not any of them). I've been told by many that it's normal, but I need to know how to stop it. Not all 8 year old girls are disrespectful and bratty right?