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.
Families come in all shapes 
and sizes these days.
.هناك عائلات من كلّ الأنواع هذه الأيّام
Heutzutage sind Familien
eben vielfältiger.
Hoy en día hay familias de todo tipo.
Hoy en día hay familias de todo tipo.
Une famille recomposée,
ce n’est plus si rare.
Oggigiorno, ci sono famiglie
di ogni forma e dimensione.
Nos dias de hoje vemos
vários tipos de famílias.
В наше время семьи
самые разные бывают.