I don't understand why they would be against counseling, that is ridiculous! Also, some of her allergy issues could have contributed to her anger issues, between getting that under control, and the counseling there is a chance drugs won't even be discussed. If it is the drugs idea that has the other family members so upset, I would assure them that you will only resort to that after trying all other avenues to help her.
Good luck!!
Oh, and here is an article on food allergies and anger in child -
http://longtimemother.hubpages.com/h...Child-So-Angry If you Google it, you can find more.