I think there are extremely exceptional circumstances which might warrant no punishment, but they would be incredibly, incredibly rare.
For example, siblings which were kept 100% isolated from the world/locked away, abused themselves, and brainwashed from an early age thst it was the 'right' thing to. Perhaps in that sort of truly exceptional case, there should be no punishment, but then perhaps it shouldn't be prosecuted in the first place.
Or if he had MH problems or learning difficulties which fell short of him not being legally responsible, but still to the extent it would be unfair to blame him.
But barring those sorts of circumstances (which don't seem to be the case here) , I can see very little reason why this boy shouldn't be punished.
However, we clearly don't know what the 'exceptional circumstances' are, and it's possible that some of us would be of a different view if we did. I very much doubt they just relate to his future, background etc.