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.
Didn't Japan used to consider the firstborn
 twin to be younger in ancient times?
ألم تعتبر اليابان أنّ المولود الأوّل
بين توأمين هو الأصغر في القِدم؟
Im alten Japan galt doch der Zwilling,
der zuerst geboren wurde,
En la antigüedad consideraban
menor al primero en nacer, ¿no?
¿En Japón no se solía considerar
al primer hijo el menor?
Tu sais qu’à l’époque,
le jumeau qui naissait en premier
Nell'antichità, in Giappone il figlio nato per primo
non veniva considerato il più piccolo?
No Japão, não é o gêmeo que nasce primeiro
que é tido como o mais novo?
Помнится, в древней Японии из двойни 
первенцем считали младшего.