Kotoha's mother and father adopted shoui after his parents died. If I recall there is a scene where yukino is talking about that in waki's arc. I understand where you're coming from though. I thought the same thing but read it over again.
Ah ok, thanks, I forgot about it.
lol yeah shoui and kotoha have some messed up genealogy going on because on top of being adoptive brothers, there's a bit of generation overlap between them.
so i guess it's just easier to call them brothers, since they were raised as such
Is it some mistake of translators?
***I think there are some SPOILERS here
Since the beginning there was told, that Shoui is Rikiichi's grandson and Kotoha's brother.
But Shoui is son of Uta, Rikiichi's sister (= R was his uncle) - and Kotoha is Yukino's son (= he is real Rikiichi's grandson) and his father was brother od Shoui's father. Shoui and Kotoha are not brothers at all.
I know that it's no big deal for a story, but it bothers me - is it some translator's mistake or just mangaka changed her mind in last 2 volumes?