I agree with Lady Pi Phi. This is the bride's [and groom's] special day, and she gets to choose who is in her wedding party, not her friends, family, or sorority sisters!

Being that the bride felt that she wanted you to share in the joy of the wedding as a bridesmaid, maybe it'd be okay to talk with her just so she has someone who can listen to her. Is she not okay with this? Or has she accepted that there will be one more person in the wedding party...or two, considering there will be an equal number of bridesmaids and groomsmen?

Either way, I think it'd be a good thing to talk it out. The only people allowed to have their way on this day is the bride...and maybe the flower girl if she decides to be tempermental.