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.
But if memory serves, weren't we 
both born on November 3rd?
،لكن إن أسعفتني الذّاكرة
ألم يولد كلانا في الثّالث من نوفمبر؟
Wenn ich mich recht erinnere,
Però, se la memoria non mi inganna,
non siamo nati entrambi il 3 novembre?
Se a memória não me falha, nós dois
nascemos no mesmo dia, em 3 de novembro.
Если мне память не изменяет, мы оба
родились третьего ноября, верно?