Adult PC Game: Mariko-sanchi no Seijijou -Oba-san wa Boku no Mono- by Willow Soft Archive

The main character is Takeru. His parents are now abroad and he temporarily lives at his uncle's house. His uncle rarely comes back home, but he enjoys his life there with Mariko, his aunt, and Akine, her daughter. He, however, gradually falls in love with Mariko though he knows it is forbidden. One day, he accidentally sees Mariko masturbating. He can't control his libido anymore and finally sleeps with Mariko....