My Stepmom's Daughter Is My Ex

The Irido family go to the countryside to visit relatives. There, Mizuto shows Yume the first book he ever read on his own, his great-grandfather's autobiography. Reading it prompts Yume to ask him what kind of person his first love was. The description he gives, that she was someone who loved to laugh, leads her to believe his first love must have been none other than his second cousin, Madoka.