Only a court can officially change the custody of your nieces. If you are concerned about your mother's actions, you should consult with a family law attorney to assist you in getting named their legal guardian.
If the children have been left with you then you can petition the court for custody. Your mother likely doesn't have standing to file suit. Beware she may elicit the help to CPS to get temporary custody. I'd speak to an attorney if you're truly concerned.
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