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.
Not much we can do now
 that we share a family name.
ما بيدنا حيلة بما أنّنا نتشارك
.نفس الاسم العائليّ الآن
Wir haben halt denselben Familiennamen.
Was sollen wir da machen?
Ahora compartimos apellido.
Ahora compartimos apellido.
On a le même nom, maintenant,
faut faire avec.
Non abbiamo molte alternative, 
visto che adesso abbiamo lo stesso cognome.
Temos o mesmo sobrenome agora,
não tem o que fazer.
А как иначе,
раз у нас теперь одна фамилия?