The problem is partly in your head. Be proud of how you are--not embarrassed to be yourself. Take it gradual, let them see you in a slightly femme outfit. Build up your confidence by going out and not worry about who sees you in slacks and a wig. After a few weeks someone will say something--and you just respond with yeah, that was me. I have had a slight thing for girly clothes since I was 6.
You can get new friends--you can't choose your sister. Be gentle. Be pround of yourself. Hope for the best.