Legal Question in Family Law in Florida

If he's the legal father then she would need his consent for minor child name change.

Within the first few weeks after a child is born, the state will accept in a new name change form from the mother. After that, it depends. If your brother has established his father's rights, either through a paternity case or through a child support case, the mother must serve him with a petition for name change. Once he receives the name change petition, he will have the right to appear at the name change hearing and object. If his father's rights have not been established, then she will be able to change the child's name without his permission.

If he is not the father, she does not need his permission.