Foster Sister problems

I am not sure if this is correct board to be on, but I am an older sister to a Foster girl. My parents have recently taken in a foster girl who is quite a handful, and I am looking for help. She is 13, but she has Mental retardation, an is on an 8-9 year old level. She has been in a group home since 1995, and the ties with her real family have been completely severed due to substance and sexual abuse.

This girl seems to be genuinely sweet, and she tries very hard to impress everyone, but sorry about this "She is so annoying!!" I would not worry so much, but I have an 11 year old and a 3 year old, and my sister has a 3 & 6 year old. This girl is not trustworthy at all, and she is very manipulative and she lies all the time, to make it look like she is the victim. She has the little ones wrapped around her finger, and believing her. However, my 11 year old is very intelligent and has saw all the way through this girl!

I want to give her a chance, and show her she can have a loving family, but I have a feeling my parents are way in over their heads with this girl. She is the first foster child they have had, and I am concerned. My mom knows of these problems that are happening, but she just doesn't seem to understand that she has to lay down the law, and not let this girl get away with everything. Does anyone have any suggestions to help improve this transition? Thanks!