Based on your scenario, feel free to claim both children. The ex-husband's new wife has no right to claim your wife's child.
If the step-mother does try to claim that child and initially succeeds (she files before you), the IRS will eventually contact both of you (by letter) to figure out who gets to claim the dependent. The losing party will then be liable for any additional taxes, penalties, and/or interest as a result of the false claim.
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