Soukan Yuugi / Sexual Pursuit

The main character of the anime is Yuichiro-san. He loves games very much and, while waiting for a new game to be released, decides to invite his stepfather and his own sister, not even suspecting what this might lead to. At first, the innocent fun slightly excited the participants, but by the time of the climax, Yuichiro threw a real party with his stepfather and sister.