Momoiro Renge: Shimai to Tsumugu Ecchi na Kankei

Based on the erotic game by Ammolite.

Yahiro, who is the head of the shrine despite being a student, was feeling unwell. The reason is that the miasma that flows through the city is gathering in his body. He is afraid he will die if he doesn't do something about it, so he decides to purify the miasma by consorting with a chosen priestess, who turns out to be his own blood sisters, Maina and Suzu. He stops himself, saying that he cannot hold his beloved family, but the miasma causes Yahiro to go berserk and he attacks the two bitches right in front of him...

(Source: parabook, translated)