gibo to oba - soshite yuujin no haha

The main character's mother eloped with someone in the past, and he can't trust women since then. But he longs for mother's love.... One day, his father gets married to Aiko, and his new life starts. But Aiko's younger sister, Kaori, appears in front of them, and their peaceful life changes gradually. Kaori says to him, "You want to shag with your mother, don't you?", and gives him a blowjob. He is confused, and finds out there is a secret desire deep inside his heart. What does he want from his mother...?