We don't know about Kousuke's (big bro) secret because we get very little from his POV, or with him talking about himself... We do know he has some sort of secret from chapter 20 (near the end), but even in that context it's kinda hard to guess what it could be.
Maybe, just maaaayyybe, some of the stuff in that chapter can be read as him being gay for his friend, accepting it as something impossible. And it wouldn't be that unthinkable for the author either, so...