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.
that would be considered
 normal among siblings.
.قد يُعتبر تصرّفًا عاديًّا لدى الآخرين
auch wenn es klare Schikane ist,
kein Grund zur Beschwerde sein.
que sea normal para los demás.
que sea normal para los demás.
tant que l’acte en lui-même
est normal pour des frères et sœurs.
che gli altri considererebbero normali.
mesmo que seja uma clara provocação.
что в глазах других считается нормой.