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 we agree to walk 
to school separately?
ألم نتّفق على الذّهاب للمدرسة بشكل منفصل؟
Hatten wir nicht die Abmachung,
getrennt zur Schule zu gehen?
¿No íbamos a irnos por separado?
¿No íbamos a ir a clase por separado?
Je croyais qu’on devait
aller en cours séparément ?
Non ci eravamo detti che saremmo
andati a scuola ognuno per conto proprio?
A gente não concordou em
virmos separados para a escola?
Мы же договаривались
ходить в школу раздельно?