He hasn't told him that his father assaulted them as kids
I thought… I thought he did since he had a distraught look when he came to his brother ?
Minjae only told him that the oldest brother and the black haired brother aren't actually blood related like he was lead to believe. He had thought he knew everything about his older brother and so he was better than Minjae, since he felt somewhat similar to Minjae. Turns out they didn't even have that blood relation, so he's torn up.
Minjae didn't tell him what the secret was between the oldest brother and him, because he wanted the black haired brother to ask the oldest brother himself. It would be torturous for the oldest to tell anyone (since he's been so adamant to keep it secret from everyone) and it would be horrifying for the other brother to hear what his blood father did to his adopted children without him even being remotely aware of it.
Wait, so the black haired brother was diddling his older brother thinking they were related or did I make that part up?
As far as has been implied, there's been no brother diddling. Just an unhealthy obsession with his older brother. That idea may have come from his obsessive ideation and the way he acts almost possessive of him. It's familiar to how many Obsessive male leads act with their object of obsession. As well as the incest already present in the family from the dad's actions.
But yeah, the black hair brother is just oblivious and obsessive.
Wait I thought he knew everything? What else does he not know?