Oof, you've got the story wrong. Yuuki, on the other hand, is really kind. The younger brother committed suicide right after his talk over the phone with his brother about his matchmaking. He cleaned his room spot-free, except for the memo deliberately put in the trash can because quote he knew his brother would find it, unquote. It just happened that Yuuki found the note before the brother and hid it. The fact is that, later, the brother found the notebook that the police had neglected.
This means, the younger brother wanted to weigh down his brother with the guilt of his death. The brother, after investigation, found out that his younger brother held taboo feelings towards him, but he didn't and will never know that his younger brother wanted him to feel guilty for the rest of his life.
The ml from the 2nd couple is the worst. Like bro just take advantage from his vulnerability losing his bro and ntm that petty jealousy? Tf he already dead at least let he know the truth you mfs. I don't even like incest but hiding the note was pathetic as fck