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.
Stupid girl... She proposed 
those rules just for that?!
هذه الفتاة... اقترحت هذه اللّعبة 
!لذلك الغرض فحسب؟
Hat sie diese Regel nur
deswegen vorgeschlagen?
Maldita… ¿Por eso propuso estas normas?
Hija de… ¿Por eso propuso estas normas?
C’est pour ça qu’elle a fait
cette proposition !
Stupida... ha proposto
questo gioco solo per quello?!
Foi por isso que ela
propôs essa regra?
Так вот зачем эта девчонка
предложила такие правила?!