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.
The sense of competition 
engendered by such a rule
،بهذه الطّريقة، نستطيع التّخلّص من التّوتّر
Das löst die Anspannung,
Así no habrá tensión, haremos nuestro
papel y ocultaremos lo nuestro.
Así no habrá tensión, haremos nuestro
papel y ocultaremos lo nuestro.
Cette pression nous poussera
à faire plus attention.
In questo modo allevieremo la tensione tra di noi,
Desta forma vamos
manter certa tensão,
Так мы решим
свои разногласия,