Keita has two older step-sisters, Ako and Riko, but since they aren’t related by blood, they love him in a lustful way. After a mishap at school, Ako and Riko finally confess their love to him. Keita dislikes the thought of seeing them other then brother and sister, but as he tries to enter the same school as his sisters, he slowly becomes attracted to them.
Back in 2001, Mahoromatic was a game changer. At the time it was by far the most ecchi series to air on TV, but perhaps more amazing was that fans generally respected the series as an anime despite the unbelievable amount of nudity and service. The ecchi genre as a whole took off over the next half-decade, no doubt thanks to the (fully deserved) commercial success of this series.
Though Bow Ditama has done some hentai work, and made a name for himself with Mahoromatic, he is currently most famous for his non-nude erotic works such as Kiss X Sis. The Juuden-chan manga was basically nipple free as well, but when Studio Hibari got the green light to add nipples in their anime adaptation, they sure as hell didn’t waste the opportunity.