Koumigami ~Nageki Warau Hebigami, Imouto Naru Monomono ga Tsumugishi Folklore by Black Cyc jap

Having angered the goddess, people will face severe trials beyond human comprehension--
Inosejima Island.
On this island is the Mikasa Temple, dedicated to the service of the white snake "Kumigami", the goddess of fertility.
The tradition of the marriage of a brother and sister was passed down from generation to generation to the rectors of the temple.
They were afraid to interrupt this tradition, for fear of punishing the snake--
Currently, Kanja Ke is the priest.
His father died of an epidemic, and he inherited the temple immediately after, caring for a mother named Kiyoka, who is sick of spirit due to the loss of her father.
Continuing the tradition, he will have to connect with his own younger sister Rei.