@Wawawelon If I were to bet money, I would bet that Yamada's father or mother would come home. And that they would be in a compromising position, unintentionally.
This series has a lot of merch combined with the manga to anime polls so I can see an anime coming soon. Atleast when the industry gets back in working order after covid and such
For anyone confused by the first page: The upper halves of the panels, where Yamada is scrubbing his back, are just what he is imagining. He calms down once he mentally pastes his sister's face over Yamada's.