The Terrible 12s

My sister is 12 and her attitude is so HORRIFIC that I don't know if she or the rest of the family is going to make it to her 13th birthday alive.
What can I say? She is obnoxious. She is rude. She has taken up hanging with the kids next door, these gross little hood rats that think they are better than the neighborhood because they go to private school. She yells. She talks crazy to my mom. She talks crazy to my brothers and I. She stomps around and cries and calls us names when she doesn't get her way, then she has the audacity to turn around and ask for new school clothes. Please, not from me!
Talking to her about her miserable disposition has gotten so draining ad dramatic that we all kind of hide from her, sitting in our rooms with the door closed as she has one of her episodes.
Right now is the worst. She is actually behaving, which, in the past, has proven to be the calm before the storm. Any little twitch, any denial of a third helping of food, any request to use the phone while she is talking to one of her airhead friends, any expectation to actually do a chore, could result in full out war.
Me and my mother were at odds when I was a pre-teen (the jury is still out on whether or not that "phase" is over) so I feel her pain. At that age, EVERYTHING seems so unfair. But I am telling you this: if she comes stomping through this house again for no reason, I am throwing her out the window!