Oyako Rankan

After a fire destroyed their village and killed their parents, Mamoru and his elder sister Kotone were adopted into the opulent Kamishiro household. Years later Mamoru now works for the family as a butler. Yet the Kamishiro household holds a dark secret: its riches and influence stem from a ritualistic orgy known as the "Feast". Kamishiro ladies are fabled for their amorousness, fertility, and the unmatched quality of their offspring, and during the Feast influential men from all around pay handsomely to inseminate these peerless wombs.
Meanwhile the bedridden Kamishiro head Ikki faces exile if he can't impregnate his wife, the lovely Saya, with the next heir by the end of the month. Pressured by his clients and his scheming lieutenant Shinohara, Ikki calls a new Feast with Kotone as the offering. With clients swarming to the manor like locusts, Mamoru finds himself plunged into a nightmare. Can he protect the family he holds dear--Kotone, Saya, and his younger stepsister Ririko--from the ravages of the Feast until the end of the month?