Kiss × Sis’s story begins with Keita Suminoe, a male third-year junior-high school student studying for his high school entrance exams. He is living in a home with his older twin stepsisters, Ako and Riko Suminoe, who kiss, flash, and display their love for him in public. They help him prepare for his exams and at the same time are trying to seduce Keita to become their lover and share a deep competitive spirit in this matter. Although they share no blood-relation to him, Keita begins to become romantically attracted to his two stepsisters and initially dislikes himself for it because he want a normal brother-sister relation. Seeing this attraction, his two parents encourage him to eventually get married to one of them much to his dismay.