I am also in your situation and have been for years. My first husband had children and my current husband had children before we married. All the children have gone through this. My children from my first marriage go through this with their step father. Granted the other parent shouldn't be involving the children in the issues but the best interests of the child aren't what every parent strives for unfortunately.
This is hard to say also because I am living it with my 13 year old step son but more from my 8 year old step daughter. This is your role. It doesn't matter how long you have been with the child but more on the child. All you can do is be you and be consistent. Be consistent in your rules, discipline and mostly your love. You need to be a duck and let the negative comments roll off your back, this is hard to do. Just keep the house the same consistent that it always is and she will turn a corner and start to realize that it doesn't matter what mom says because she knows what is true.
My children get left at home or dropped off early by their father because he got a better offer. They aren't dumb they can see through whatever excuse he gives them. They give us attitude for a while but we just love on them. We tell them that we are thankful for the extra time with them and validate their feelings.
We have also used counselling through the harder parts for our childen starting about 5-8 years of age. At whatever point they go to school and figure out that not all families are like theirs. This starts attitude and questions also.
My parents divorced over 20 years ago. My mom went on and rarely discusses my father - I appreciate that in her. My father is still hashing out everything for me and I'd rather not be around him to get to hear it. I also now think that my step parents are great and rely on them near as much as I do my mother.
I wish you all the best.