No, don't mention it, she won't be able to hear you. Just take it as:
1) You need to spend as much time as possible with your daughter, as she is feeling the fact that her mom's attention is elsewhere
2) If you do end up divorced and eventually in a new relationship, your daughter may not resent having a stepmom - which is a good thing
I do wonder where she got the idea from though - does she have friends who have stepmoms? If not, I wonder if your W hs planted the idea in her head somehow. Seems unlikely but....?
