Susan

You were a guy that had a fairly normal boy's childhood, with a few exceptions. For some reason, proper manners were more enforced with you than your brother, and your mother sometimes asked you to help her with her hair and makeup. It all came together when you turned 18 and woke up as a woman in bed. It turns out you had been born with a rare genetic disease that would cause you to transform into a woman when you turned 18. You had tested positive for it at a young age, but your parents kept it a secret from you, so they tried to secretly prepare you to become a woman growing up.