The normal positive modeling does not work with ON. That is what my IL's have been using for the past two years. It has been a constant "ON please do this, not this". "Please use your manners, this is how we take turns.." Etc, etc. She was becoming so out of control, hitting, bitting, name calling, that no one wanted to be left alone with her.
I simply introduced a calm time out, delivered after two behavior warnings. No arguing, no angry words on my part, just a calm " you hit your sister after being asked to stop, now you can sit for 4 minutes." Rinse and repeat.
DH and I are the only two people who have ever made her follow through with a time out and apologize. Yes, there is a mental issue concern which is being looked into, but I did show the IL's that she could control certain behaviors. She is now much more pleasant to be around and they are all a bit less stressed.
Not negative punishment but tough love. She avoids me when she misbehaves because she knows I'll call her on it. This is child who was so out of control that DH and I stopped sending our 13 year old DD over to spend the night when she kept coming back with bruises and bite marks from ON. She didn't want to defend herself against her cousin and the grandparents just kept asking ON to use her manners and not bite but would not step up and actually stop her from hitting and bitting.