have a serious talk about the messages she sends to others about what she thinks about herself . ask her if she wants the attention of the boys because she wants them to just have sex with her, or if she wants a boy to like her because he values her personality.
our culture is terrible about sending girls messages that lead to them not valuing themselves. as parents and adults in general we need to do more to counteract those messages.
You teach them when they are young to cover up their body. As the parent you only allow appropriate clothes. I think it has to do with their friends also. If they are friends with skanks then they will want to be a skank too. It's all about teaching self respect
I don't have teens. But when I was younger my parents approved the clothes I got. If they didn't like them I didn't get them. Plus they put the fear of god in me of I went behind their backs and got skanky cloths.
Having talks about the message your cloths send.
Also telling your child no, your not wearing that. Which a lot of parents just have a hard time telling their children no.
Don't buy it. That seems pretty easy. They can buy their own cloths but how much can they buy at 14. I also wouldn't wash it, if she wants to dress like a 2 cent hooker she can wash her own cloths. Plus don't even think about calling me if you get in trouble at school for cloths. You can sit in the office, wear gym cloths or whatever.
Who controls what your 14 year old cousin gets to buy? Additionally, if she is dressing as you say, odd are good the girls she hangs out with are doing the same so maybe she can work with their moms on elevating the standards of the group.