Divorces are very hard...as are step parent relationships with children from the "other" woman. I don't want to judge you or the way you think. Or the way you feel about this little girl.
The person who will pay in this scenario is the little girl. You can be her confidant, her friend, and her cheerleader, but you cannot be her Mom....unless you for some reason have to someday adopt her because of the loss of her Mommy or other tragic incident. That doesn't mean that you cannot treat her with love and the same way you would your own child.
I am from both of my parents "second" family. I had the benefit of my Mom and Dad together. My brothers didn't. They came into this world that sucked! Their parents HATED each other. And for them, they felt like an afterthought. My brothers however had the benefit of the marriage being over and a divorce being final before they met my Mom.
My Mom became their friend, advocate and cheerleader....not their Mom. Now they call her mom (they are 50 &53) but they didn't when they were young. Their Mother deserves the title of Mommy, she did then and she does now. They call my Mom, "Mom" now because without her, they wouldn't be who they are. The title was earned and not given. Just ask them...
When you move away, that tells that young girl that, she wasn't worth being around for. When you have that baby, it tells her that "she has a replacement for her Dad's affection", When you get married to her Dad and all those changes take place, she learns a terrible and untrue thing about her...she is second, or last.
I know you love her...it is easy to love a child when you love her Dad. But will you still feel the same way when she shows her jealousy of the baby, or when she repeats to you what she hears her Mom say about you? I think that you have to remember one thing, she is the one being screwed. She loses the benefit of a WHOLE family, she loses the daily kisses from her Dad, she loses seeing him when she needs him to guide her.
Be her friend, her advocate and her cheerleader...not her Mother, that job is going to be super hard for her for much of her life...unless her parents GROW UP and act like adults, it may very well be too late for that!