Mizuto and Yume used to date, but now they've suddenly become stepsiblings. Both of them agree that they don't want to do anything that'd disappoint their newly remarried parents. However, neither of them can accept the other as being the eldest sibling in the relationship. So they both agree to settle the matter with a game. Whoever does something in conflict with what people might consider normal sibling behavior, loses. Making them the younger sibling.
You'd already forgotten, huh?
أقال أنّه نسي ذلك سلفًا؟
„Das hatte ich schon vergessen,
bis du es erwähnt hast“?
¿Cómo que ni te acordabas?
¿Que te habías olvidado?
Il avait complètement oublié ?
Te n'eri già dimenticato, eh?
Esqueceu até eu lembrar?
Без упоминания бы и не вспомнил, значит?!