There was a little girl in the KH where I grew up, she was a "mistake" and I can't really remember a week that went by that someone in her family didn't remind her of it in that not-quite-joking voice. I thought it was horrible then, and said so, but nobody listens to a teenaged girl in the Organization, even if her dad is one of the most intimidating guys in the hall. If it didn't come from an elder, it was less than worthless. Her step-grandfather turned out to be a kiddie rapist, btw.
Thank you so much for sharing your tale. Sadly, I think this dirty underseam is not terribly uncommon in high-control religions. I'm glad you have your siblings (and a therapist) to talk this over with.