I read somewhere that this phenomenon was spurred by those Mary-Kate & Ashley twins. I've seen them, while zapping. Their hair is done up by pros, their clothes are ultra-adult, they wear heels, etc., and of course, I'm sure they're changing clothes 3 times during one half hour show. I do believe it is cultural. I've seen some special reports here in France about how much dough moms are doling out for their pre-teens' wardrobes. One showed a birthday party for a 9 year old where they're all freaking about what to wear and explain how if a pair of jeans has the wrong kind of pockets or seams or whatever then that can lead to social death! Moms are spending enormous amounts of money. So culture, peers and parents are the major influence.
I was still a kid when I was 9, though I started to be boy-crazy at 6! I was 11 when I started to want to wear heals and makeup and such. But I had to sneak. I wasn't supposed to be allowed to wear hose, makeup or earrings until I was 16 (but then daddy took off when I was 13 so I got my ears pierced the next day!).
My advice is to avoid talking/thinking about how hellish it's going to be. Try to keep a positive attitude, especially with her. Don't tell her she's being moody or tease her about boys. Try to give her some special privacy (if she doesn't have her own room, then a special desk just for her, or a place in the attic or basement). Respect the fact that right now her sisters seem like stupid little pests but also try to explain that they admire her and want to be like her.
My mom bought me a book (too late IMO) in which I could write all my feelings, dream about my future, read about girls and women. She might enjoy something like that, especially coming from you.