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.
So your plan is to get close 
with the younger brother, right?
إذًا فخطّتك هي التّقرّب من الأخ الصّغير، صحيح؟
Haste deshalb den
kleinen Stiefbruder angequatscht?
Piensas llegar a ella usando
al hermano menor, ¿no?
Tu plan es entrarle usando
al hermano pequeño, ¿no?
C’est pour ça que t’as approché
son petit frère ?
Perciò il tuo piano è di avvicinarti
a suo fratello minore, giusto?
Por isso quis se agarrar
ao irmãozinho dela?
Так ты решил сдружиться
с её младшим братом?