Adult PC Game: Osananajimi ga Chichioya to Dekiteita Ken ni Tsuite (jap) by Appetite Archive

Miyabe Saya is Kamagiri Kouta's friend living nearby. She often helps out with housework at his house, which has become a father household since his mother passed away. One night, Saya stays over at Kouta's house. He wakes up around 2 AM to go to a bathroom and hears a faint voice coming from his father's room. He sneaks over and sees his father and Sayaka having sex, which makes him feel betrayed and angry, as he believed Saya liked him. Kouta's feelings of love for Sayaka turn into hatred.