it will eventually get better. 15 seems to be that age where everything is insane. after like 16 it slowly starts to reset. i kept telling my sister (who was a very disrespectfull, just horrible person) that when she turned 18 i was going to beat the daylights out of her for all of the disrespect and rudeness that my mother should have beaten her for all along (mind you i dont even spank my kids let alone condone beatings aggain you would have had to know her) well ill tell you what, when she was 17.5 her attitude quickly changed. personally i like to believe that she knew what was coming and that if she changed her attitude then she would avoid an @$$ kicking, but she finally reached a point where she realized just how messed up she was being. when that happened everything changed. on her 18th birthday i took her out instead. not that i still wouldnt like to get her back for the 9 years of crap that she put my family through but she seems to have really changed for the better.
hang in there. it will get better. you know what you can try? make her get involved with a church group. no one in my family is religious but my sister got into one and it changed everything.