Tell her that you assume her goal, as wll as your goal and your husband's goal, is for their daughter ( your stepdaughter) to grow up to have a NORMAL, wonderful life. If that is to be the case, then sooner or later she will have no choice but to wonder why her "natural" mother acted in such an angry and divisive way (fill in your own details here, obviously).
Unless, of course, your husband's EX's intent is for the daughter to grow up with a poor sense of what is real, and what love means, in which case she is well on her way.
Or something like that. Appeal to the EX's future self; appeal to your step-daughter's adult health.
Probably too idealistic an approach--I don't have any real experience in these matters.
-------------------- --Boothby
My goal: Once a day, make someone laugh so hard that their soda comes out their nose