Responses
I think the family do several rituals during their childhood to trap the divide being inside the ML, the process causes the ML to be sickly and have mood swings. So the MC comes into the picture and shoulders the "misfortunes, hurts and sickness" for the ML.
Oohhh, yeah, that clicks!
ML being a divine being and MC being his scapegoat. Is the ML cursed or sumthing? Pretty sure they have a past life which they were entangled and it's a mess. I wouldn't be surprised if the ML told his parents he wanted an adopted brother and choose the MC on purpose.