Best Woman...?

So my fiance's sister happens to be his best friend. I've known of this the entire duration of our two plus year relationship. However in recent weeks and in our discussions of marriage and marital plans she has referenced herself on more than one occasion as the best woman. At first I kind of laughed it off thinking it was just something she was saying to be funny. But now that she said it a number of times and he has also made reference to it I'm beginning to think that they really believe that this is somehow appropriate. I totally disagree with the idea of another woman other than myself standing next to my future husband calling herself the best woman at my wedding. I really don't care that it's his sister, I just think that it's beyond outrageous to have another woman at your ceremony standing at the altar next to the groom calling herself the "best woman". Am I overreacting or is it really just that ridiculous that she could think that she should also be standing beside my man?