Based on the erotic game by Cattleya.

Yohei happily lives with his mother, Kaho, and his cousin, Fumika. But one day, Kaho tells Yohei that Kaho is actually an older sister of his father, not his mother.... She also tells him that his mother passed away long time ago. Yohei is shocked to hear that, but he starts seeing Kaho as a woman, not as his mother. Days later, a woman, Ikue, moves to a house next to his. When Ikue first sees Yohei, she suddenly starts crying and Kaho turns pale. Ikue tells them she is a relative of Yohei's mother and she wants to accept him as an adopted child. Of course, Kaho and Fumika turn it down.... From that day, their lewd and erotic life starts....

(Source: ErogeShop)