You say that social services says she can control her behavior...but has she really been evaluated and diagnosed by a psychiatrist?  If not, that is where you should begin.  She could have bipolar disorder or other mental illness that will not get any better until diagnosed and treated with medication as well as therapy.  I understand why she has been in the care of others because violence like that is  unacceptable in any home.  But there are ways to approach it besides sending her to a different caregiver, ways that would ultimately be of more help to all of you.  And that is with professional help.

Maybe you have gone that route with no clear improvement?  You might go to the bipolar site as well as the depression site, and see what the experts say.  I hope your daughter can be helped soon and that someday she can be re-introduced to her family of origin.  I know you hurt for her and are trying the best you can to find the right kind of help.