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There is no legal recourse if it was a casual situation and she was not holding herself out as your child's legal guardian or something similar to that.
However, you could insist with your ex that the children call her by her first name and not call her mom, and that in the future she inform others that she is not the biological mother of the children should it come up again.
You and your husband have a legal obligation to work together in the best interests of the children and having to call a second woman mommy would likely not be in their best interests. In addition, if she every attempts to sign documents holding herself out as the children's mother, you could file suit for misrepresenting her identity.
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