Andrea-
My difficult child took risperdal. It worked well for him. We did eventually stop it due to weight gain, but it has many positive benefits as well. We wound up stopping the risperdal and started taking a true mood stablizer. I have seem many kids do quite well with risperdal.
Chasing a sibling with a knife is dangerous and quite serious. Many would even suggest to bring him to the hospital with this type of behavior...Please put away and/or lock up your knives and other sharp objects until difficult child is more stable....Even though the doctors are suggesting strong medications...it sounds like your difficult child really needs it.
Good luck with it.