*** TIE BREAKER RULES IN SHORT ***
If 2 taxpayers both claim the same dependents then the person with the HIGHER income wins and the loser has to pay back the money they got from claiming the kids. SO you said she did not work in 2021 thus it sounds like she filed to get the CTC on the kids and possible the EIC based on 2019 earned income so they may have a lot to lose. If you can talk to the other parent and come to a reasonable agreement for both of you.
