Dude. Even if they're not blood related, they're still brothers. They were raised as brothers. Heck, he called their parents mom and dad. Like how his younger brother called them mom and dad. Its gross.
Even if they were raised as brothers if both or one of them didn’t think they were brothers- can’t really say much- cuz Yuai never thought of A as a brother.if i remember correctly- or he did at first but that changed or smth. That’s the whole reason why they started arguing kinda cuz Yuai never thought of A as a brother and A felt betrayed by that.
Plus as I said before this is fictional
He quit as a doctor for AAAAA AHHHHH would’ve been nice if he stayed as a doctor and had lots of powerful connections with more good people and he could’ve used that but if he didn’t have the connections to protect A as a doctor.
Also did Yuai choose to be adopted or did his grandpa not want to take care of him??? well considering the complicated family relationship no wonder grandpa didn’t take him☠ bro was probably too busy or didn’t even know until later or smth- but idk
Also if it helps- I see them as more of friends to lovers kinda troupe. Because there are some scenarios where childhood friends A and B end up together. And A has seen B since B was in their mothers stomach. And A has been with B since B was a baby. A and B viewed each other as siblings but as they grew one of them or both of them started falling for the other one (or each other)
(I’ll be referencing friend 1 and friend 2 as A and B where friend 1 is A and friend 2 is B)
Plus this is fictional and they're not even related☠
Now if they were related I would be so unhappy and ugh cuz they’re blood related but if they’re not blood related and just adopted then idc. (My mindset.)