Step Families: Favoritism
Before you speak to the stepmother about it, you should speak to your former husband. Often times the husband/ex-husband can be the buffer between the former wife and the new wife, especially when it comes to parenting/step-parenting issues.
If you feel he isn't supportive or understanding of the issue, then you should speak with the stepmother. Perhaps she feels competitive toward your daughter and not toward your son, perhaps your daughter reminds her of you and your son doesn't. Before you speak to your ex-husband or his wife, you might consider her point of view so that your request, on your daughter's behalf, is better formulated.