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.
Maybe he just thought 
you were annoying?
أواثق أنّه لم يجدك مزعجًا فحسب؟
Vielleicht weil du so nervig bist.
¿No será que eres muy molesto?
¿No será que eres un pesado?
C’est toi qui es lourd.
Non è che gli stavi troppo addosso e basta?
Não é porque você é um cara chato?
Может, ты сам тот ещё надоеда?