I agree with stephbac. In the meantime, anytime she is doing this kind of behavior, you may wish to put her in her room (or a room by herself) and tell her when she is done acting this way, she can come out. She must apologize before she can interact with the family. It sounds like they have tried to do everything but refuse to give attention to this behavior. I know this is a worrisome time, so act quickly. Please, please, don't let the 18 month old out of your sight.
You are in my thoughts and prayers.
I have a 41/2 year old and the behavior you described is not normal. I have a question, when they say that they have tried everything, I wonder how consistent they actually were. At this point, if it were my daughter, I would enlist the help of a professional. Ask her pediatrician if he or she has any recommendations for therapists in the area. They may even want family counseling along with individualized counseling for their daughter. Good Luck!!