C.L.
I don't have much experience in this area or much advice, except that I think you are right much of it probably has to do with her age. My oldest son just turned 13 so I am just starting the teenage years. Maybe she's a little jealous of her little brother? With three parents in her life it is probably possible that she is trying to manipulate the three of you to her advantage. I wanted to say that I thought one of the other posters was a little harsh, and I applaud you for working with her mother and having a positive relationship with her. I think that is very mature and could have nothing but a positive effect on your stepdaughter. I have no idea why you would want to assume she is "the enemy," and I think that would only make things worse for your stepdaughter. Blended families can be difficult and it sounds like you are a very busy woman, but maybe you could get some counseling to give you some additional ideas of how to work with your stepdaughter and not let it interfere with your relationship with your husband either. Good luck.