I wasn't there, but honestly it sounds like an accident to me. It sounds like the kids were playing and Rupert got excited (likely because of the whistling noise) and jumped up and scratched your brother accidentally with his nails. If it was a deep cut then I bet his nails are too long and sharp. I do not see ANY reason whatsoever to even consider putting him down for something like that.

I doubt Rupert even knew he did anything wrong, dragging a dog back to the "scene of the crime" doesn't do any good...if they are not caught in the act, they are not going to "connect" the reason they are being punished. I bet he didn't even have a clue why you were mad at him, and he certainly does not have a clue why he is now banished to the garage.