Children identifying as non binary can lead to tricky situations

My youngest daughter is getting married this summer and has asked all her sisters, sisters in law and nieces and nephews to be in her wedding party. All is fine EXCEPT that one niece who is 13 years old seems to be identifying as non binary and is, according to her mother, too uncomfortable and anxious to wear a bridesmaids dress. The bride wants all the girls to be in bridesmaids dresses (they are very tasteful). Although I can understand both viewpoints, I ultimately feel that a child shouldn't be dictating what she wears if she wants to be part off the bridal party. Similar to if she was on a dance team or a gymnastics team, she wouldn't be able to wear her own costume if she didn't like the team costume. I also think it's good for children to learn that sometimes they need to dress up for an event. Any ideas how this problem might be solved? It would be terrible to cause any family rifts over this and quite frankly, I sometimes get a little tired of everyone having to be so woke.