How do I deal with this brat?

Okay so my younger cousin (shes 13) is becoming one of the most: obnoxious, slutty, rude, and meanest people I know! she used to be sweet and Innocent, but now she; makes fun of girls (including her friends) , shes acting like a whore; her dad took her to jersey shore, and she got a pair of short that are so tight they look like under where, and they say "Italian *****" on them. and she had her younger half sister take a picture of just her butt in them, than posted it on facebook, i didn't say anything mean i just said "yummy" (kidding course). she wears so much makeup that people have came up to me and told me to tell her that she wears too much. and she acts like such a brat; she calls her mom a b*tch, and if her mom tells her to clean a mess up in the house she just says "why are house is sh*tty anyway" and worse of all all the boys in my grade use me to get to her...it makes me so mad, they always telll me "your cousin is so hott" i just cant deal with it anymore what should i tell her?btw she isn't a very understanding person