Custody

Amber • Baby boy due September 10th. 💙💙
My brother has been to court for custody of his child. He has her every other weekend, and mondays. Today he should be getting his daughter till monday, but the childs mother is trying to withhold her for the weekend but its his weekend. I guess my question is, is if she can legally do that, and if not does anyone know the proper way to handle the situation?