I think it starts with giving yourself permission to to be YOU and not your sister. I am in a similar situation. My older brother has always been the person, according to my mother, that I should be striving to be. It hurts to feel you aren't accepted as you are, but looking for acceptance from others causes you to lose your freedom. You become trapped doing things just for acceptance.

The way out is to start accepting and appreciating yourself. Notice all the things you appreciate about yourself. Tell yourself that you will trust yourself and exercise the freedom to be who you are. You are beautiful and wonderful the way you are - don't let them take that away from you.