Overhaul 叔母穴～オーバーホール

Yuya lives with his father and mother. He enjoys his life with Kazuki, his close friend. But one day, Yuya sees his mother, Mizuho, and Kazuki having sex.... Days later, Chihiro, Mizuho's younger sister, visits his house. His evil desire arises and puts some medicine into a coffee he has just served to her....