Why child services, with the amount times they have been involved, have allowed to the stay in their custody astounds me.
This is a tough one to answer and there is no simple answer as to what is better for the child and what is better for society. All you can do now is try to help as best you can. You are getting different opinions here and hope it helps you think about what is best in your situation.
Until your mother contacts the authorities about a missing daughter there is no problem. You would know better than us if she is likely to do so. Even then you are unlikely to get into trouble as the police would seemingly understand the background.