Lord help me I can relate. When I married my wife her son, now my step-son was 10 and I dealt with the same things. Its normal, and always remember, no matter how "mature" they act they are children.

It would appear that the dad has created a triangle with the lot of you. This isnt good. He has sent the message that he sees you both as equal, this isnt the case.

I am going to tell you what I did. I approached my wife and calmly expressed to her how I felt and what I observed taking place. You have to choose your words carefully so he does not get defensive and throw up a wall. Just let him know you need him to set a boundary with his daughter where your relationship is concerned.

He should also have reassured his daughter that she still has a special place in his life and that you wont replace that. As far as the discipline goes, he needs to be the one to do that. That is my opinion. That doesnt mean that if she is raising hell you dont do something but as far as correction goes, I feel its the place of the biological parent.

This way the child doesnt resent you further and it lets the child know that dad isnt going to tolerate his daughter disrespecting you. Its a tough situation, but it does get better. Once she knows your safe she will calm down, but right now in her mind your trespassing on her "turf."

I think if you can keep in mind that it has nothing to do with you and that she is insecure of her relationship with her dad it might make it less frustrating. The dad really needs to step up though and have a talk with his daughter and do it with you present. She needs to know you are both on the same page and that her behavior will no longer be tolerated. Do you see marriage in the future?