@Kirin-kun That was my guess. He doesn't even really remember the brother at all which means he could either have been super cute and naturally easily confused for a girl, or was outwardly portrayed that way in the first place.
I feel like the story wants you to believe the blonde is the brother just by how outwardly flamboyant they are, but I'm hedging my bets that the girl just got super outgoing and the boy is the calm one.