How can we get my great-grand-daughter back home?

My daughter took custody of my great-grand-daughter and refuses to return the child despite the fact my grand-daughter has since had another child, cleaned up her life and has provided a nice home and is more than capable of caring for the child and ignores the fact that the child wants to go home with her mother and brother. Instead my daughter is clinging to the child with hopes of adoption and to gain child support. She claims she's looking out for the best interest of the child yet has nothing to do with the other grand child. There is much more than what meets the eye in this case and the child is suffering being denied her rightful place with her family as well as my grand-daughter and her son being denied their rights.