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.
Don't let it bother you. 
They're not worth the time.
.لا تدعي ذلك يزعجك. لا يستحقّون وقتك
Ich brauch einfach nur
nicht mitzuspielen.
Ignóralos. No valen la pena.
Pasa de ellos. No merecen la pena.
Je les écoute à peine,
je m’en fiche.
Non te la prendere, non ne vale la pena.
Basta eu deixar eles falando
e não entrar na conversa.
Не бери в голову,
они того не стоят.