Seikishi Ricca no Monogatari: Hakuyoku to In'yoku no Shimai

The holy sunlight, illuminating those who live on earth... The stronger that light is... The brighter it shines... The denser the darkness becomes... The farther the shadows fall... Two lonely sisters, one chosen by God, the other transformed into a demon.. A fairy tale about their wicked fate adorned with lust and obscenity... The younger of the two, Ricca, is endowed with the power to transform into a holy knight, and sets off on a journey in search of her abducted sister. Conquer the powerful monsters that stand in the holy knight's way through a wide range of combat actions! [From DLsite]