Imouto Jiru: ep. 1
Akira's father, an archaeologist, married twice. Poor boy now has to live in a house with his new stepmother and three grls Yuki, Nana and Miu. Akira's father leaves for honeymoon with his new wife and makes an archaeological find, so he makes a decision to stay on site so he can supervise. Akira is left at home all alone with four women. Eventually he finds himself in bed with his stepmother and runs into a box that contains a scroll and three flasks. As his fathere tought him the Sumarian language, he's able now to read on the scroll, and all of a sudden he realizes he's in a contract with the goddess Inana, who orders him to gather the girls love juices in the flasks before the next full moon. If he doesn't obey, or else a something horrible will happen.