My dd is now 12, and ever since the others started getting trendy, she has move din the opposite direction!
She actively chose ds hand me downs etc.
Last year, in year 6, they went on a residential after SATs and I was concerned that she looked rubbish, and they all looked trendy, but I didn't want to squash her style and her choice, so before the residential we went shopping.
I asked for advice on here, about where we could go that wasn't girly and might have stuff that fit her.. We went to both Mountain Warehouse, and H&M. (boys dept, not girls!)
I was very clear that she could choose what she liked, but encouraged her to try on lots of stuff.
We also talked a lot about colours, and what works and what doesn't (she does have a fondness for green, and then wears 3 different clashing greens) so we found other things that would go with her favourite green stuff.
It really helped her to bridge the gap. She started to work out a proper style for herself which is much better than the big brother's hand me down look. She still wears jeans and fleeces, but now has a funky hair cut, and some bright T shirts, so she looks like it is a chosen look.
We went out this half term actually and she chose some really good stuff (suits her) but she pretty much only buys from the boys department.
I actually think that helping her to find her own style has enabled her to stand up and be herself. She is the only girl in her class at secondary who wears the uniform trousers, rather than skirt, and there are only 2 girls in the whole of year 7 with short hair (dd and her friend).
By contrast her younger sister is the queen of twirly skirts. No hand me downs going in that direction!