Marikosanchi no Seijijou 茉莉子さん家の性事情～伯母さんは僕のモノ～

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 sister. He, however, gradually falls in love with Mariko though he knows it is unachievable. One day, he accidentally sees Mariko masturbating. He can't control his libido anymore and finally sleeps with Mariko....