Based on the erotic game by Rose Crown.
The Sagemiya family is cursed in that its members are sexually aroused in the presence of another of the same bloodline. Sagemiya Misaki and her elder twin, Shun, are both disgusted over this and have sworn to resist the calling of their blood. However, Shun loses his composure and goes to their elder sister, Kagu, for release. After Misaki finds out, she too loses her resolve and seeks out Shun.
(Source: Anime News Network)
