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'll be smooth sailing if I can get 
to her through him, y'know?
سيكون الأمر أسهل بكثير إن تقرّبت 
منها من خلاله، صحيح؟
Über den Bruder zu gehen
ist doch gar nicht so dumm.
Será fácil si hago buenas migas con él.
Si hago migas con él, será pan comido.
C’est plus facile comme ça.
Andrà tutto liscio come l'olio
se riuscirò ad arrivare a lei tramite lui.
É mais tranquilo chegar
através do irmãozinho, certo?
Через него всё гладко пройдёт.