
They aren't real brothers. There's a message by Ogawa Chise at the back with the drawing of the other guy who has a twisted relationship with his father.
"I thought of making them real brothers but I stopped myself. I chickened out."

They r actually brothers since they have d same father.

I believe you are right, they are non related step siblings. different mothers, remarriage, their parents raising them to view each other as brothers, and her last comment of her chickening out, it all pretty much shows me that they are not related by blood.
and the one with the real brothers is in RAKUDA TSUKAI TO OUJI NO YORU. there is also a "prequel" from the brother's point of view and gosh...compared to them this couple is totally tame.
I'm confused because they have different mother and the lil bro's mother is remarried, so it's not actually an incest because they aren't related? or maybe I get it wrong?
but I still love it i'm all in for this kind of stuff but srsly the older bro is so thin he need to eat properly. I mean c'mon lil bro you seems to care so much about him but you failed to see how thin your bro is or maybe you secretly enjoying this??!