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 either one of us may yet slip up
لكن قد يخطئ أيّ أحد منّا
aber früher oder später wird einem von uns
doch was rausrutschen, oder?
pero en algún momento uno meterá la
pata y hará algo impropio de hermanos.
pero en algún momento uno meterá la
pata y hará algo impropio de hermanos.
ma ad un certo punto, 
uno di noi potrebbe fare uno scivolone
mas vai chegar uma hora que
um de nós vai se descuidar
но в какой-то момент
один из нас может сорваться