Mom is being a jerk over my bridesmaid's hair.

I have this sister, Laura, and she is one my bridesmaids (and a great one at that). The thing is that she has grown her hair out for years and it is all the way to her leg now. She always braids it, so it's a little shorter when it's up. My other sister (also a bridesmaid) has the same hair color, except she has shorter hair. And my mother keeps pressuring Laura to cut it so as not to draw attention away from me on my wedding day. I do not want Laura to cut her hair unless she wants to, and my mother's snarky remarks on Laura's hair is annoying for all of us. What should I do?