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 thought of being your 
little sister creeps me out!
!فكرة كوني أختك الصّغرى تجعل بدني يقشعرّ
Mir schaudert bei dem Gedanken,
deine kleine Schwester zu sein!
Pensar en ser tu hermanita me repugna.
Pensar en ser tu hermanita
me da repelús.
Il solo pensiero di essere
tua sorella minore mi dà i brividi!
Me dá calafrios pensar que
sou sua irmã mais nova!
Быть твоей младшей сестрой это кошмар!