I think he's just snapped completely.
He felt intense guilt as soon as his little brother came, and then he started to break down when he left him. When his brother showed back up and he started to treat him like his father had (especially the snapping of the finger) the MC's mind kept breaking down further.
In his mind, he wanted his little brother to stay pure and innocent, wishing that he had been the one to kill their father and to hurt others instead of the little brother so he could stay innocent. When his brother gave him his happy smile, the MC further broke down and then resigned himself to stay so his brother could reclaim his innocent.
In a way, the mc is allowing himself to be a sin eater, and by giving in and being his brother's pet, his brother gets to be 'normal' and happy.
However, the mirror thing is a very good observation - I wondered if that mirror wasn't there when his father had the room. There snapping of the fingers absolutely has a hypno thing going on (MC blacked out and was suddenly tied to bed, for instance) and it's possible that mirrors also might have a roll in the hypno thing. It's possible that blondie told him 'what a happy family we are' and the mc on some level accepted that as his truth, and that played into why he stayed.
Interesting how that mirror also made appearance of Theo mom they were fuckin in same room where cendric killed his father and his father used to be in that mirror exists there.
Bro i get the feeling something happened at the end w/ the mirror bc that would explain why the mc is staying w/ him outta “his own” will. Like how twisted would that be oouuu