Letting your little brother take the blame for what you did is a debt you’ll feel for a long time.

Letting somebody else absorb a punishment you earned does not end the matter, it moves it inside you. He takes the consequence and forgets it faster than you expect. You keep the part that does not wear off, which is knowing what you let happen to somebody who looked up to you and could not argue back. That is a quiet weight, and it tends to stay.

You can put it down whenever you decide to, including years later. Tell him first, plainly, that it was you and you should have said so, then tell whoever did the punishing. Both conversations are far shorter than the carrying.