Am I wrong asking my son to keep his pregnant sister in his wedding?

My son is getting married. They asked my daughter to be a bridesmaid and she accepted. She is now 12 weeks pregnant. They recently told her that they what her to step down and be a reader. They say it would be in her best interest for comfort and health-wise. She will be 7 1/2 month (34 weeks) She agreed to it being it's their wedding but is very hurt. She is considering not going to the wedding. As a mother of 3, I don't see a problem. I feel its her way of saying I don't want a pregnant person in my wedding. My son sides with his fiance. They have a 10 month old daughter. I wasn't overly joyed finding out. Not because of the baby but I question her motives. I prefer marriage before the baby. Like the way my daughter has lead her life. Married 3 years and now having a child. Now I have an angry mother of the bride telling me I'm not a good mother. Not showing unconditional love to my son, ruining her daughters happy day, etc. I have two children who's feelings are hurt and I love very much. If I'm a bad mother I failed at teaching my son to stand up and support your family. Am I wrong asking my son to keep his pregnant sister in his wedding? What is the right thing to do to keep peace in the family without taking a side?