@elegia Kei found out cos of the couple's phone calls at night. Thin walls made Kei hear it all even if she didn't want to. (or was it that the siblings are sharing a room only divided by a curtain, forgot)
@karman19
For a sec I was worried as well but in the first chapter it states that he's 18 y/o so it's likely that it's simply their last year of highschool. (inb4 tragedy tag gets added or some shit :<)