I'm of the theory the eldest son is a mind-reader like Anya, either as the cause of her experiment or the result of. With the father's scar, it might be the same for him as well, though we have a better case for the eldest son. He might have been separated for the mind-reading experiments so the wife wouldn't notice and the eldest wouldn't know what was being done to him was wrong. Or, he was young enough when it started that he freaked out his mother.
I completely miss the scars. I have to reread the chapter and wow, it's like he has a brain surgery/alteration.
I feel like Damien's mother has to hold back from showing her children affection because she's scared of the father. Maybe he believes in some shit education method like "separate the child from the mother to make him tough up". The brother's academically excellent yet like a robot.