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 winner gets to decide how the 
younger shows it every time.
"سيتمّ تحديد "الأخ الأصغر
.أو "الأخت الصّغرى" في كلّ مرّة
Wie genau der kleinere Geschwisterteil
es zum Ausdruck bringt,
El ganador decide qué debe hacer el
perdedor para demostrar que es el menor.
El ganador decide qué debe hacer el
perdedor para demostrar que es el menor.
On réfléchira au gage en question
le moment venu, si ça te va.
Il vincitore deciderà ogni volta in quale modo
il perdente dovrà dimostrare di essere il minore.
Vamos decidir na hora o
que o mais novo deve fazer.
Победитель решает,
как проигравший должен показать,