Akuma no Hanayome

Deimos was once a handsome god. He loved a beautiful goddess who returns his sentiments. The only problem was that she is his sister. For their crime against nature they were struck down out of Olympus. The brother is now a demon and the sister a rotting corpse at the bottom of the ocean. Deimos now has to choose between his sister and her living human incarnation but the sister is jealous, while the girl is horrified and unsure of just what to make of her situation.