Before your sister can answer you scream "GIRL ME!". When everyone look at you strangely, you explain "All I ever wanted was to protect my sister's happiness, and she seemed happy to have me at her side. So if from now on I need to be a girl to stay at her side, fine, I'll be a girl! S'not like I ever cared much for being either gender, anyway."
Girls started discussing it among themselves, some astonished by your declaration, others terrified of the idea of someone who didn't care about being either girl or boy, and the rest thinking it was a trap.
In the end, the leader of the girls, who call herself their "Queen", decide to submit your future to her "subject's" vote.