Why is my teenage sister such a brat?

I disagree with the belief that teenagers are automatically little brats. I was never like that when I was her age. There may be something in her life that is making her like this. I know that this is no excuse for your sister but, still it may be something to consider. There may be something that you do not see. I also think it may be that teenagers see all their friends and people on tv, ( even adults telling them that being a brat and an inconsiderate little twit comes with being a teenager) acting that way so, they think that they must do the same. It gives them license to be that way.