I think the best thing you can do is just be civil and kind to her and take a limited role with her. Hopefully doing so can help DH to pay more attention to the mean-girl sypmtoms she's displaying and deal with them appropriately.

Well, it depends. Is your DH custodial? If so you can expect the little monster to make your life a living hell, progressively more as she gets older and decides she's the wife, not you.

If DH is non custodial, don't sweat it. Be as polite as you can muster while she's there and wipe the house out with lysol when she leaves. Invest in a few boxes of wine and a good lock on your bedroom door.

I'm going through what your going through my sd is 17 was 10 when me an my gf got together. She is roughly the same as you desribed with your sd. long story short with out the help of your SO nothing most likely will change,like i've told my gf time doesn't change children discipline does, time only makes them worst because they become more set in there way (SO and SD). i told my gf we can marry once SD moves out ,lets hope that helps motivate my situation. Hope yours works out more smoother than mine.