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.
It seems he's finally realized the true 
nature of the rules I laid out..
.يبدو أنّه أدرك أخيرًا الطّبيعة الحقيقيّة لهذا الاختبار
Er scheint gemerkt zu haben,
Creo que se dio cuenta
del truco de mis normas.
Creo que se ha dado cuenta
del truco de mis normas.
Il a remarqué ce qui se cache
derrière cette règle.
Pare si sia finalmente reso conto
della natura di queste regole.
Parece que ele percebeu a
verdadeira natureza deste jogo.
До него всё же дошёл
истинный смысл моих правил.