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.
Wanting to bury the hatchet is completely 
normal for siblings, don't you think?
إنّ الرّغبة في نسيان أم ما شيء عاديّ
بالكامل بين الإخوة، ألا تظنّين ذلك؟
Sich nicht streiten zu wollen
ist doch ganz normal.
Querer hacer las paces es normal, ¿no?
No querer que nos peleemos
es normal, ¿no?
Dans une famille,
c’est normal de pas vouloir se disputer,
Voler seppellire l'ascia di guerra
è del tutto normale per dei fratelli, no?
Querer parar com desavenças é completamente
normal entre irmãos, não acha?
Почему же нежелание ссориться
это проигрыш?