If at all possible, completely ignore her. Unless she's physically damaging something or someone, just get yourself away from her, and call her dad to deal with her. If you have her for extended periods alone, look into making alternate arrangements. If she doesn't listen or at least comply when you want her to leave your general vicinity (ie. go to her room), she's pretty much set on taking you to battle. This is going to get much worse if you stay engaged as a parental figure to her. I would let her parents do the parenting, with the hopes you can preserve a little of your pride. I had a 10 year old SD that did the same thing with me. I loved her and was patient, and tried to always conduct myself as an adult. She's now turned into a 17 year old SD that has told me "go f*ck yourself", and "you're not my mom, bitch!", in front of my DH, DD12 and DD2. Isn't she precious? How cute.

How many years in to step-mothering are you?

Unfortunately you can do nothing my sd10 does that to me when she does that I have her dad deal with her and if he's not home I call him to deal with it I've told him if he doesnt dealI with it I will refuse to watch her sometimes its waited til he gets home and others during phone call good luck its really important you you have dad backing you up when she acts up Take privaleges away its gotten better at our house but not clear from it yet

I would ignore her too. I hate watching my SD7 and SD9 because they don't listen. They don't throw fits, they just ignore me. So I ignore them. The odd time I do watch them, they do whatever they want (as long as no one is in danger) and I just don't really care. DH and BM don't discipline them, so I figure why in hell should I?