My friends hate my sister and are always rude to her...?

Ok my twin sister is a butch (im straight and really girly) but of course I still love her.Shes been a for as long as I can remember she told me she had a crush on a girl when we were 4.We are 13 now and all of my friends are rude to her.When she comes around they look at her like shes a freak and like "eww". One time I had just changed clothes and my sister came up and just gave me a half hug and my friend was over and she said "eww you need to change your clothes now" I didnt though and they are always saying things like "she likes you...." and "shes gonna you oneday" and I dont have the courage to tell them off I wish I did because I love my sister and she is my best friend but I just dont have the courage and they are the only friends I have.How to either get them to be nice? or what to do