In reference to a will, is a step daughter treated as if she is my daughter?

In a properly made will, a relationship does not matter. As long as he indicates her legal name, and that is not confusing or possible to be confused with someone else, it should not matter.

The rules vary from state to state, but I would ensure that the will is crystal clear as to who gets what. If your husband wants to make sure that somebody does NOT take anything under the will, the will should say exactly that. I usually put a clause in the will as well that states if someone contests the will, they only get $1. Your husband should seek counsel from an attorney in your area with expertise in estate planning.