And here I was thinking the brother was just one of those events we probably missed because this game requires you to repeat actions after it tells you nothing is there to find some of them. Nope! It's a plot point.
I should clarify this because it's something I made up and it's not as obvious as I meant to intend -- there's no actual post credit scene, it's something I completely made up.
The game just seems to drop all mentions of Kayo-chan's brother altogether as far as I can tell. I am curious whether I missed something though.