Akuma no Hanayome: Saishuushou

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