I couldn't agree more. SIX months is a loooong time to have worked with a behavioural problem with no results. And even worse, it is a LONG time for the other, poor little dcgirl, to be constantly victimized in what should be a safe place for her to spend her days.
You need to terminate. Your responsibility is to ALL the children in your care. Obviously the child either has some serious mental problems OR everyone is NOT on board as they claim with regard to disciplining this behaviour or it would have STOPPED by now. If you say that YOU have been trying to stop the behaviour then it has to be that they are not doing their part at home with regard to acts of aggression.
She needs to go.