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.
If I wasn't like this before,
،إن لم يكن هذا حالي من قبل
.فأكيد أنّها غلطتك أنّني هكذا الآن
Wenn ich früher anders war,
dann liegt das wohl hundertpro an dir.
Si no era así antes, te aseguro
que es culpa tuya que lo sea ahora.
Si no era así antes, te aseguro
que es culpa tuya que lo sea ahora.
Si quelque chose m’a fait changer,
ça ne peut être que toi.
Se prima non ero così,
allora lo sono diventata per colpa tua.
Se eu não era assim antes, então
certamente é sua culpa que sou agora.
Даже если раньше и была другой, 
то изменилась я по твоей вине.