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.
That's not true. I was a 
nervous wreck on the inside.
.ذلك ليس صحيحًا
.كنت متوتّرة للغاية في داخلي
Oh, si, j’essayais seulement
de ne pas le montrer.
Non è vero. Dentro ero un fascio di nervi.
Isso não é verdade.
Eu estava tremendo por dentro.
Вовсе нет. Меня внутри
аж трясло от волнения.