Mashou no Kao

Keiichi Hayama is an ordinary Japanese high-school student, who has lost his parents and lives together with his foster mother and her daughter, which he considers his own sister - or so he thought... Because all the boy can think about is sex, and the two women he lives with are both very pretty, not blood-related to him, and also sex-hungry. To make things even worse, an exceedingly well-endowed exchange student comes to live with them. In short, what follows is a series of increasingly erotic encounters, which pretty much sum up the plot of the game.

[From Moby Games]