Abusive Biological mother wont give consent for immigration with kids please help if you can.?

Ok before every one yells at me, My husband and I have custody of his two minor children. We live in South Africa, he is a USA citizen, our daughter was born in the USA his kids are South African as am I. The biological mother has a checkered history and we have FOUR Forensic professionals that have confirmed that the children are abused, (physically, emotionally and ually) whilst in her care, all four professionals have stated that she is beyond Rehabilitation. Our judicial system is retarded and after 2 years and near bankrupcy she still has the right to see the children without supervision and remove them from our care. We are worried sick and feel that immigration is the last resort, how ever she wont give us consent to immigrate with the kids. We will return annually to see my family and the children will tehn be able to see her as they do have the right to know her, does any one know of a LEGAL loop hole for us to move as a family.