In line with the game by Rose Crown.

The Sagemiya family is cursed in that its members are sexually aroused in the presence of another the exact same bloodline. Sagemiya Misaki and her elder twin, Shun, are generally disgusted over this and still have sworn to face up to 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.