I see the infraction as mostly being the breaking of a house rule against hitting and I would try to make it clear that he's being punished for that, even if his younger brother was acting poorly. I don't think the provocation needs to weigh very heavily.

With the younger one, I think getting hit was probably punishment enough. However, I would emphasize that if you act like a jerk you might get hit (not that it's right or fair or just or anything, just that it might happen), rather than emphasizing that his brother was wrong to react that way.

In short, you don't have to take sides. They can both be in the wrong.