I urge you to take the family outing. Not going is too far away from when he hit to be a teaching moment. And it punishes the whole family.
A more serious consequence won't teach either. I would focus on teaching him. Have you tried listening to him and not lecturing? The immediate consequence of being separated from his sib is a natural consequence. I would send him to his room and tell him he can come out when he's able to say he's sorry. Do this over and over. It's the consistency that teaches.
Changing the consequence puts the focus on the consequence instead of finding out what is going on in his mind and helping him learn how to handle anger. Role play situations in which he's apt to feel anger. Model for him what to do when he's angry. If you hit when you're angry he's doing what he's learned to do. I'm not saying you do. If you yell when you're angry he's learning it's OK to lose control when he's angry. What do you do when you're angry?
A calm and consistent, "go to your room until you're able to say you're sorry" will eventually work when combined with teaching him what he can do when he's angry.
There are many good books written for children about anger. You can find them at the library as well as a book store.
After your SWH: All the punishment in the world will not help him learn appropriate ways to deal with his angry feelings. Punishment instead of discipline will only make him angry. He may stop the hurting of his brothers. However, his feelings will go underground and come out in a different way.
You've punished him and it didn't help. I don't understand your logic in thinking more severe punishment will help.
Also, I see this behavior as common and not serious. He has feelings and doesn't know how to appropriately handle them. This is an opportunity to teach.
You said you know why he does this. If you could tell us, we might be better able to talk about effective ways of discipline.
Above all, I would take the family outing. It has nothing to do with his behavior. The outing is for the family and will help build stronger bonds.