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.
Did she really tell everyone
 we're stepsiblings?
هل أخبرت الجميع أنّنا أخوان غير شقيقين بالفعل؟
¿De verdad anduvo contando
que somos hermanastros?
¿De verdad ha ido contando
que somos hermanastros?
C’est elle qui a révélé
notre lien de parenté ?
ha davvero detto a tutti che siamo fratellastri?
Ela espalhou que somos meios-irmãos?
Уже всем рассказала,
что мы теперь сводные?