If you had no role to play in your stepdaughter's life so far and that you are attending the wedding only as your husband's current wife then you should dress up like any other guest. If the mother of the bride single handedly raised her daughter then it is absolutely fair that she wants to be the only mom at the wedding. In such case it is best to shy away from all the limelight and assume a seat in a row or two behind the bride's and groom's parents.