“If you have an incredibly beautiful younger sister, anyone would become a siscon.” Usami Aoi is a severe siscon, considering his mysterious-looking younger sister, Runa, to be the most beautiful in the world. When Aoi sprains his wrist, he starts receiving support from Runa and even begins to sleep together with her. Concealing her brother complex, Runa finds it increasingly difficult to suppress her feelings, gradually turning into a yandere. Updates with one or two chapters every day. The story primarily revolves around sweet developments between the protagonist and the heroine, without uncomfortable twists.