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.
So she told people I'm younger, huh? 
That girl...
أخبرت الآخرين أنّني الأصغر؟
...يا لها من
Hat sie denen etwa gesagt,
dass ich ihr kleiner Bruder bin?
Y encima les dijo
que soy el menor. Maldita…
Encima les ha dicho
que soy el menor, ¿eh? Maldita…
Quindi ha raccontato in giro
che sono il minore, eh? Maledetta...
Então ela diz que sou o irmão
mais novo? Aquela garota...
Рассказала всем, что я младший?
Вот же...