Personally, I hold nothing against you and your role as stepmom. My DH is a wonderful stepdad who would be crushed if the older two thought he loved them any less than his biological child. I agree a step can be just as loving as a real parent. BUT, I do not agree that a step is the "same" as a parent. You are NOT a parent. You are a STEPparent. In my opinion, that means you get to love the child as much as the parent, play the role of a parent, but always be willing to step aside to make room for the parent each and every time. I would imagine stepparenting is a pretty thankless job, and it takes a strong loving person to do it effectively.
Have you considered how the mom in your case feels about all of this? You say it's a control issue, but I have to wonder if because you do have such a great relationship with the child and your hubby supports you being his equal in the home, is there any chance you are making mom feel undermined and replaced? Not saying it would be intentional, but is it possible? Perhaps if you could address this and work to make sure she knows she will always be MOM and you have no intention of filling that role, she wouldn't feel the need to flex her parental muscles at you.
JMHO, of course.