There is probably some resentment that has grown over the years on her part about another woman helping raise her child, I think that is pretty normal in step parent situations. That is probably what compels her to interfere and harrass you.
I think about all you can do is continue not to deal with her, and every single time she gets upset about it, your husband needs to re-enforce to her that all issues dealing with the child are between him and her, period. Hopefully she will just give up on it.