adoption of little sister

we live in georgia and have been taking care of my husbands little sister. she has been with us for almost two years. she is actually in dfacs custody she has been placed in our home, like a foster family. the parents were on probation for her not going to school. they were recently arrested and charged with possession with intent to distribute on two different occasions. they thought that if they signed over their parental rights then they would be off of probation, so they called the caseworker the father went to her office and signed the papers, then the caseworker and the father went to the jail and the mother signed hers. The father then took the papers of to the probation officer. the question is we cannot leave her in dfacs custody so we will adopt her. Is there anyway that the parents can still be made to pay the court ordered 150 dollars a month even after we adopt her? any information would be helpful the dfacs office has not known what if anything we would be able to do since we are family.