With all due sympathy for your family situation which can not be an easy thing to deal with, "her" does not apply to your sibling.
History: I had a younger brother who was homosexual; I never knew until after his death at 41 in San Francisco. I have a nephew in his early twenties who is dressing and behaving female. I do NOT call him "her" or "she" as doing so would denote acceptance of his illness as OK and not in need of treatment.
she is a girl that dresses and lives as a guy so yes she is a She.....I try not to refer to gender at all when talking to her which is not often because she knows that I feel being transgendered is a form of mental illness.,