Elf no Kyoudai ~Netori Haramase Koibito Koukan~

Crow is a student of a high-class magic school and Crea, his older sister, is a teacher of the school. They are from Alhaim family and love each other. However, Alhaim family and Milan family suddenly reach a reconciliation and it is decided that Crow marries Ursula, a daughter of Milan family. One day, Crow sees Ursula and Leonid, her younger brother, having sex at school. Crow suggests to Ursula, "How about we marry outwardly...?" It's just a disguise marriage.... Like this, they start living together. Their marriage life, however, gets bolder and bolder....

From ErogeShop