Especially since we're not normally distant siblings either...
[Considers it.]
Let's just say Zero threatened us to keep it secret. You said you remembered more people than that man from what happened nine years ago. Isn't it strange they too acted like they didn't know us or each other? "Zero" must have worked something out to prevent everyone from talking. Saying that is the case with us too isn't much of a stretch. To begin with, we can say that we simply don't remember why we were pretending not to know each other yet, just that we know we were pretending.