Yeah. That's what happened to me. Raised him from the age of 2.
There are so very many variables in this step parent/child equations that it becomes almost impossible to predict a solution. In fact, even the problems are unpredictable. I know men who are very close to their step children; men who do a fantastic job; men who have been total failures; women who get between the child and his step father, usurping the man's position; women who have maintained lifetime relationships with step children for whom they no longer have responsibility; and just about everything in between.
Three things I encourage:
1) A sense of humor
2) A sense of fairness
3) The right amount of patience (That one's a little nebulous, I know.):)
But DO NOT use the child in any way to get to his Mommy, OK? :)
I agree - I would also advise that you and she have an understanding going in what your obligations and expectation will be with regards to the child - AND that you do not allow any situations where one of you says no - and the other says yes to something and the kid is able to play you against each other.
Stand up for what is right, even if you have to stand alone.