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.
By the day, perhaps,
 but that's not everything.
.وفقًا لليوم ربّما، ولكنّ ذلك ليس كلّ شيء
Es stimmt aber nur
der Geburtstag genau überein.
El día no lo es todo.
El día no lo es todo.
La date est identique, certes,
Forse il giorno coincide, ma il resto no.
Só o dia é o mesmo.
В один день, но не в одну секунду.