Nightmare x Sisters ~Ingoku no Sacrifice~

The protagonist, Shindou Setsuna, is a female student attending school. She spends time after classes and free days doing holy sister activities with her sister Yuuka. Yet these girls are not just ordinary holy sisters; they have another side. This side is the exorcist one—they cleanse demons that possess people under the cover of night.

Lately, demons have been appearing more often, so the protagonist thought that it was a bad omen. And just then, her sister, Yuuka, was taken prisoner by someone and vanished without a trace. Setsuna searches for her traces with her childhood friend Kitakawa Touma and the holy father, to whom she is so indebted.

Furthermore, the one who kidnapped Yuuka went even for Setsuna. And this was an introduction to the endless hell of debauchery...

[Translated from DLSite]