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.
but every now and then, you go and 
do something outrageous, don't you?
لكنّك تقومين بشيء مشين من حين لآخر، صحيح؟
aber manchmal steigerst du dich in was rein
und machst was total Beklopptes.
pero de vez en cuando
haces algo muy atrevido.
pero de vez en cuando
haces algo muy atrevido.
ma ogni tanto fai qualcosa di stravagante, no?
mas vez ou outra você faz
coisas impensáveis por impulso.
но нет-нет да и
выкидываешь такие номера, да?