If your sister is displaying emotional reactions to her physical state, seek mental health input and arrange an evaluation by a mental health professional. The family doctor can be a source of referral and recommendation.
Good things come in small packages!
Size is not important at all, what you are on the inside is what is important. She will realize that in a couple years. She is at the age where most kids can find something about themself to fault. I bet if she asked her friends if they could change something about them what would it be and she would get an answer from them all. It is a part of growing up. I don't have any suggestions. Just wanted to comment.