Based on the erotic game by Alice Soft.

Akito Natsuki decides to spend his one month long summer holiday in his hometown after his workplace closes down for renovations. With no money and nothing to do he decides to take it easy.
At home Miyuri, his sister-in-law he was once fond of, and Sayoko who always treated him like her own son, were waiting for him.
Akito tried to hide his feelings as always and acted out the part of the brother-in-law, but after a slip of the tongue his real feelings are revealed. Then his stagnant relationships with Miyuki and Sayoko start to develop again, and eventually he gets deeply involved with them.

(Source: Getchu, English)