Koumigami ~Nageki Warau Hebigami, Imouto Naru Monomono ga Tsumugishi Folklore~

There exists an island with a Shrine, where a Snake God is worshiped. The deity demands that the siblings of the Shrine marry each other, and, if the precept is broken, a curse supposedly will fall upon the family. Protagonist's father did not follow this custom and married a woman, who was not related to him by blood. He fathered a son (the protagonist) and a daughter. After their father dies, their mother begins to act weird, while a mysterious man shows up on the island, asking about the old family custom and the curse....