Why is my sister such a brat?

I think your sister needs to chill out and be thankful for what she's got! A lot of girls would kill to be able to even attend the concert! If it was my daughter, she wouldn't even go to the concert after acting that way! No way. Then she'd realize how good she had it and wouldn't act the same way next time an opportunity arose. Maybe your mom needs to put her foot down and be respected by your sister! Your mom sounds like she tries really hard and doesn't deserve to be treated that way.