Smile, I don't think you made a mistake by posting. Debs came here looking for help and by offering her our suggestions, maybe she can find some help in the form of something one of us says.
This girl is causing a terrible problem for Debs and her family. The girl is obviously angry and by the looks of it getting angrier by the day.

Debs,
Not that it's any of my business but has this child recieved any counseling?
Does she act like this at school? Or only at home with you?
Does your school offer counseling? Maybe an outsider can help with this situation.
I'm sorry that she is putting such a strain on you and your family.