I am sure my opinion is unpopular but at 14 I think the best thing for her would be to participate in the wedding.
You mentioned that you think her anxiety has to do with the wedding itself and not the circumstances to your marriage. In this case, I think this is a learning opportunity.
Her life will be so much better if she can participate in the celebrations of the people she loves in the future. This is a small, small wedding with very limited social threat, very few strangers present and surrounded by people who love her. Its the perfect opportunity for her to learn how to cope with feeling uncomfortable in order to put the needs of someone she loves before her own.
I don't think you should force her if she's really resisting cause that will cause you both undue stress before your big day, but I do think you should do everything in your power to encourage her to be involved. Anxiety is a compounding force - a vicious cycle; anxiety causes avoidance which causes more anxiety and so on.
Could she have a grandparent on either side? A friend?
Could she be at the very back? Could she be sitting at the front and then just stand with the others at the end? Could she meet before hand with the celebrant to make her feel comfortable?
All this said, I think its really worth speaking with her very earnestly to understand if she has any reservations about the marriage. Even if she loves her step dad, I agree with the other posters that the wedding might symbolise something scary about the future to her that she can't quite articulate. It may be driving the anxiety further and could be dispelled by some reassurance from both mum and stepdad.