Did you skim read? They explained it well that the main character doesn’t remember anything but he stays with him because he’s the only one who knows about his childhood (and because he needs a place to stay) and after awhile he remembers some stuff and witnessses some things that link to the black room and the mirrors.
I think it’s the last chapter they explain it but when they were young they despised the parents so much that their reflection in the mirror killed the parents for them and now if they ever wish for someone to leave them alone (or a person they hate) then their reflection will kill that person
(The mirrror version isn’t them exactly but rather the version that makes their deepest wishes come true)

That was like taking an acid trip. Even with the story ending i have no clue wtf is going on. Idk know what the plot was. I wish i didn't pick this up. The smut not even worth it compared to my disappointment..