Did anyone ever really believe you would have someone in your wedding party that wanted to look better than you?

I know my younger sister. She always needs to be the center of attention. At family gathering she is always in the skin tight clothing and she is always drawing attention to her body. She is loud and obnoxious and she prides herself on being the skinniest person in the family -- to the point that when she heard that I had lost a few pounds... she went on one of her many starvation diets. I really think she will try to outdo me on my wedding day. In fact, i Know she will. This makes me so upset. I know she will tan herself so that she is more bronze than me, she will whiten her teeth, get the hightlights, lose more weight oh and she always flexes in pictures. This really upsets me. Did anyone ever have anyone like this (a sibling perhaps??) who made them feel the same way?