I always hated being called a girl when I was school-age because I don't identify as female and the term "girl" implies many things socially OTHER than what biological gender you were born such as.. likes pink, giggles, loves to shop... etc and I do not feel that that term described me accurately. I don't deny that I am biologically the female gender but "girl" is used in a much different way, imo.
I don't think that is your sister's issue though
*edit* Although, on second thought, it is possible that the above is the exact opposite of your sister's issue if she does indeed identify with femininity.
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Non-NT something. Married to a diagnosed aspie.
Nothing is absolute.