I should be a psychic!

I don't believe she can. Has your brother gone to the Police and said, "Here's the situation"? Sometimes they'll act, sometimes they won't. I believe he has grounds for an emergency Family Court hearing.

I think the father should go get his kids. If this woman refuses to allow it, let him call the police and show that he is the father and that this woman has no right to keep his kids.

If the police won't help (and they may not), then go to court for an emergency custody order.

While I understand that there may be no specific court order stating custody, it is implied by the fact that he has been ordered to pay support. That order establishes as the legal father and entitled to custody if the primary custodian is unable to care for HIS children.