First of all well done and thank you for taking care of this poor child.
It sounds like this little girl has been throuh so much
And now not only is she apart from her mum but has moved to a new invironment. Does she know that shes going to see her mum again? Has any one sat down and spoke to her. I think she needs a really big hug, lots of love and time to adjust to the changes in her life. Of course there has to be rules. Dont listen to the mum about hitting, she obviously needs to go to some parenting classes. Time out is prob the best idea, and how about rewarding her when shes really good. Try to give her a good routine. Good luck