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 wants a normal brother-sister relationship. Seeing this attraction, his two parents encourage him to eventually get married to one of them much to his dismay.
Kiss×sis began as a seinen manga series written and illustrated by Bow Ditama which originally began as a one-shot printed in January 2004 in Kodansha's seinen manga magazine Bessatsu Young Magazine. The manga began regular serialization in the same magazine on December 11, 2005. The manga was transferred to the Monthly Young Magazine on December 9, 2009 as Bessatsu Young Magazine becomes a monthly publication instead of a bimonthly one. The first bound volume was released in Japan on September 6, 2007 published under Kodansha's KC Deluxe imprint. As of November 2009[update], five tankōbon volumes have been published.
It was announced in June 2008 that an animated adaptation of the manga series would be produced. An original video animation episode produced by the studio Feel and directed by Munenori Nawa was bundled with the third manga volume released on December 22, 2008 as "episode 0". Another OVA episode, "episode 1", was bundled with the fourth manga volume released on May 22, 2009. The third episode, "episode 2", was bundled together with the fifth volume of the manga on November 20, 2009. A fourth episode is scheduled to be released with the sixth volume of the manga on June 4, 2010. At the end of the third episode it was announced that pre-order sales will end a couple months before it goes on sale, and there will on be an exact number of copies produced to match the pre-order sales. The series uses two pieces of theme music. The opening theme is "Honey Boy for Two" (ふたりの♥ハニーボーイ, Futari no ♥ Hanibōi) by Ako and Riko while the ending theme is "Starry Sky Tale" (星空物語, Hoshizora Monogatari ) by Nana Takahashi.