I think the misfortunate part is that kousuke(ooshiba big bro) is actually the one in love...

I think the misfortunate part is that kousuke(ooshiba big bro) is actually the one in love with setagawa. And hasekura noticed that there will be a lot of drama between setagawa and kousuke because of that as kousuke will be in denial and shifting it to setagawa is the one who is inlove with him instead of being true to himself. Hence why hasekura said to ooshiba that setagawa is already a misfortunate person as he is the one being pulled left and right.
I mean setagawa admires the brother and adores ooshiba , hes very supportive(or has a fuck it attitude) about hasekura and ooshiba relationship but is happy as long as oshiba is happy with hasekura. hes just very satisfied with his life right now living in peace these past few years not including all the random stuff kousuke made him do (note in the prequel setagawa was dragged when he arrived to ooshiba house by kousuka and then dressed like a women just to get into a bar with kousuke to have a chat with asaya(ooshibas big sis) ) there is a lot of turnmoil on the road for setagawa and kousuke to finally be together . Please note that i havent seen the raws for hitorijime my hero so i have no idea how the story will go as the chapter moves on. As i can only read the eng translated chapters . Frankly my japanese reading skills sucks bad.