Call me crazy but I think the brother was obsessed w MC
I could see that too. Smth like the brother was sickly and sheltered and kept away from the outside world so when he met MC that treated him kindly he became obsessed with him, but MC liked ML and brother became jealous and did smth that resulted in serious injury/death and blamed it on/ guilted MC which caused him to run away from ML.
That one panel MC hallucinates or perhaps remembers beating up the brother and the way the Brother haunts him I feel like, like everybody else he was obsessed with the MC too
But I agree with you clearly MC didn't do anything he's just too kind and nice and feels guilty of something he never did
I think it may be the brother was "obsessed" with ML because eof that toxic family and when ML started getting close to MC in their highschool years, the brother tried to "drive off" MC, but MC couldn't help his feelings for ML and the brother caught them kissing or something so the brother did su*c*de and the MC thinks it was his fault..? Or maybe no self death but the brother found out and ran into traffic and got hit by a car, and the MC was like "omg how could this happen" and the brother goes "this was your fault" and dies.
Either way I don't think it was an incestuous relationship between ML and his brother.