While there are similarities in law from state to state, it would be very risky for a Georgia lawyer to offer advice about Maryland domestic law. If your brother is in Maryland, you will need to contact a Maryland lawyer for advice on this issue. Hopefully an interested Maryland lawyer will respond to your question.
I am providing general information on the law only, and no attorney-client relationship exists by virtue of this exchange. You can not rely upon this as formal legal advice. If you require specific legal advice, you must hire an attorney and then your communications will be protected by attorney-client privilege, and you will have the benefit of an opinion of counsel. Call 770-753-9995 for a free evaluation.
If you are in GA and your brother is here now, you should bring your custody documentation to a Family Law attorney in your area. Whether the father can regain custody will have to be decided by a judge. If you and your brother have lived here for over 6 months, a GA judge should (but may not) decide GA has jurisdiction, but if not, be prepared to go to court in MD.