So my sister is becoming trashy?

ever since my 21 year old sister met her best friend she has become trashy and kind of b*tchy. she treats me like crap and she has let herself go physically. i think she's actually considered "obese" now. she's gotten a bunch of piercings in her ears, which i have o problem with except that now she is getting her naval pierced, she's talking about a tattoo, and she wears the most revealing clothes ever. one day she was wearing a white, totally see through, off shoulder top with a pink polka dot bra under it and it looked so trashy. and then just the other night, when the family went to a wedding reception with LITTLE KIDS there she wore this really thin, see through yellow dress that was sooooo low cut that like her boobs were really hanging out and it looks so trashy and there were little kids there and this was a family event!!!!! my sister still lives at home and drives our parents car, eats their food, uses their electricity etc. and she acts like she owns the place, treats me like crap and disrespects my parents, who refuse to put down there foot about anything that she does!!!!!!!!!!!!! and my parents aren't the type who will kick their kids out after a certain age. at least not this child. she is the oldest so she has gotten everything and my parent's will never kick her out. did i mention she's living rent free? they will let her live at home forever if she wants. What do i do? oh i forgot to mention that i'm her little sister, youngest kid of three, i'm 16 and my brother (her little brother) is almost 20.