I feel for you, you have got to feel kind of helpless in this situation. Its wonderful that your (?) mom has been there for you.
Its kind of crazy to ask for more information - but all you have talked about (with good reason) is his outbursts against his sister and you. These outbursts may have nothing to do with ADHD, but more with the situation he is in. Or they may have a lot to do with ADHD. Tough thing is that they kind of need to be handled differently depending on what is going on with him.
First question is how long ago was he diagnosed with ADHD. Is the doctor giving you any help with ideas on how to work with him? What kind of a doctor is this? pediatrician, psychiatrist, psychologist? Did the doctor suggest medication and is he on any meds.
Is your son is preschool or kindergarten? What do they say about his actions?
Does he have all the other symptoms of ADHD, or is it just the physical response? Or, I guess, is the physical response the thing that bothers you the most?
These are the basic questions. I am trying to figure out if he really does have ADHD, or he is just responding like some 5 year olds do to his living situation. Or does he have something else like Sensory Integration or is bipolar - both of which have very similar symptoms.
For the time being. My main suggestion is to really make sure that you know and understand ADHD. I highly suggest that you buy the book (fairly cheap through Amazon), "The ADD/ ADHD Answer book," by Susan Ashley. It will give you help in many, many ways. One of the (many) things that it goes over is how to punish and/or change the child's behavior. I do wonder how you and granny deal with his misbehavior. Its hard to believe but many of the old standard punishments people used to use with their kids - actually don't work with ADHD kids and even make the situation worse!
I know that I haven't been too much help here - well the book is really, really worth your effort to get - but I really need more info.