Responses

Because A think they're real brothers... But the AUTHOR made it clear they're not related by blood... You can read through the descriptions she used like "they are nothing alike" "they just grew up together"... It has the same implications the author used to show how his brother looked at him differently... Just read it again until you grasp that...
I wonder what kind of story people are reading, because it's not this one...
It's been a few chapters already that shows the way he looks at A and it's already made clear that they're not blood related... Why are people so confused? The official translations are there...