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.
We consider the firstborn 
to be the eldest now anyway.
.نعتبر المولود الأوّل هو الأكبر الآن في كلّ الأحوال
Heutzutage ist bei Zwillingen
der erstgeborene doch der große!
El pasado no tiene nada
que ver con el presente.
¡¿Ahora no es el primero en nacer
el considerado el mayor?!
Aujourd’hui,
c’est le premier jumeau l’aîné, non ?
Adesso il primo nato
non viene considerato il maggiore?!
Não consideramos que o primogênito é
o que nasce primeiro entre os gêmeos?
Сейчас-то первенцем
считают старшего ребёнка?!