Akuma no Hanayome

Deimos, once a handsome god, loves a beautiful goddess who returns his feelings—but she is his sister. Punished for their forbidden love, the siblings are cast down from Olympus: Deimos becomes a demon, while his sister lies as a rotting corpse at the bottom of the ocean. Now Deimos must choose between his sister and her living human incarnation. His sister is jealous, while the girl is horrified and uncertain about what to make of her situation.