What is the best way to make my sister realize she shouldn't be dressing so sluttish?

Depends on how old she is, and her maturity level. If she's just a little kid, you don't want to scar her. But if you think she's at an understanding age, show her what could happen. You can find rape statistics, and there's things all over the internet about "what rapists look for" and how guys'll think they can take advantage of her. I know how you feel; my sister seems like shes headed for that road. Its not easy to watch. Hope everything works out.

My sister is exactly the same, but she's old enough to understand, if just seems she doesn't care.
What i advise is that you all sit down and explain to her what can happen. Suggest alternative clothing but be fair. Short skirts are fine, but suggest she wear leggings or tights with them. If she wears low cut tops, suggest a cardigan or a jacket that could cover up a little more. Is she's not yet old enough to understand all the consequences of sexual contact, tell her and get her some protection to carry around with her just in case something does happen.
Take some time for her to show her how to add make-up subtlety instead of plastering it on.
And also hammer down that the less she wears the more intrigued a man will be, it will show them she's not plastic and easy and therefore she will also get respect.
To be honest, it's not your place to tell her what she should be wearing, it is her life and it's her choice, but there are ways to slow it down a little if that makes sense.
Good luck with her :)

ok i say tell her exactly what could happen. tell her not meaning to scare u or make u mad but. You could be sexually hurassed raped. Violated and people could take u the wrong way. you need to let her know dressing that way may be cool to her friends her crush whever thinks its cool But it could really hurt her as a person to were that. I know going threw same stage with my little sister.