Ahhh, that's exactly the same year that my sister got into that kind of stuff. She suddenly began demanding only specific designer brands of clothing, had to get a specific style of shoe, listened only to specific music. All of it was because that's what the other girls were doing and she wanted to fit in with them.

Has your sister been doing what she's doing for long? If it's only been in the past year or so that she's been acting the way you're describing she might become more tolerable by the next school year.