There are few guidelines for stepmothers and stepfathers about how to understand child development or to parent effectively, instantly. They jump into middle of a kid's development and try to make sense of it. Usually with little support.
When you fell in love with your husband, I am sure you had great hopes of creating a happy family that included both of your children, but as you are discovering, blending a family is often easier said than done.
What message does the word "stepmother" convey more clearly than "You're NOT a mother"? And why does a father's new wife need to be contextualized in relation to the mother at all? It declares a winner... as if society doesn't already halo moms over non-moms enough.
The greatest challenge to a second marriage is not usually direct conflict with the stepchildren, but rather, conflict over the parenting of the stepchildren, especially if the new husband suffers from Guilty Father Syndrome.