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HelloThis is Samuel.First of all, the Mother can let your niece back in. However, your niece should consider filing an ex parte to get the child custody on a temporary basis. She can go to the clerk of the court where the child is living and tell the clerk she needs to file an ex parte to get Temporary custody of the child until there is a full hearing.
She should tell the clerk she is the Mother, the ex has the child and she is concerned for the welfare and safety for the child because he has also illegally kicked her out of the home. If there is any mental, emotional or physical abuse in this situation to her she should put all of that in her Petition.