From what i read, the older brother always craved for their parents attention. So he always do everything the parents want and try to be the best. While the younger brother craved for older brother's attention (maybe the reason he's obsessed sexually).
After the brother left home, younger brother maybe got insomnia from not having older brother around. Since the insomnia got worse, parents sent older brother to take care of younger brother.
Since the younger brother is obsessed, he wanted older brother to stay. While he's like sexually obsessed too, he drugged older brother using his medicine to relief himself.
Why did older brother took the med? Cuz his habits is taking supplements. He thought those were his supplements since younger brother said so.
The ones with red backlight is his dreams.
The ones with normal lighting r real stuff happened.
After the older brother didn't take the med, he realised the reason he feels off r cuz of the younger brother. So he left to korea. And on 3.3 chap, younger brother is searching for his older brother.
(Mind u, this could be wrong, but this is what i thought)
Just love how author portrayed those feelings. I'm just- so happy with the ending