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.
What on earth is she thinking?
ما الّذي يدور في ذهنا يا ترى؟
Was denkt sie sich dabei eigentlich?
¿Qué tendrá en la cabeza?
¿Qué leches tiene en la cabeza?
Qu’est-ce qu’elle a en tête ?
Che le passa per la testa?
Que raios ela está pensando?
Что она задумала?