I am glad you are putting the couples wishes first. This is your child's day and what loving parents do is exactly what you are doing. They put their needs and wants last and they put their children's needs first.
Your other child is not capable of being so selfless and should be allowed to boycott the wedding if he or she wishes. There is not a thing you can do about this, except what you are doing, and that is by attending the wedding and refraining from contributing to the bad mouthing and negativity you are behaving in a rational, adult and loving manner. Your family and friends will lionize you for this behavior. You are taking the high road and this means you are above the nonsense.
The one point I will disagree with you on is this, you say:
This will be very uncomfortable to all in attendance other than the father.
I know there are many petty people in the world but the fact is that you are going to be pleasantly surprised to find that most people will behave well. Most people will not be uncomfortable in the least and in fact will be relieved if the sibling stays away. It matters not how small minded your family members might be, they will behave well. No one wants to publicly show themselves as unevolved so if your childs father attends they will behave.
I know you think you are immobilized but by setting the example that you are, by showing all that your child is the most important person on their wedding day you will set the tone for the event. This is not only a true sign of your mental health and rational ability it is something we used to call class and manners.
Two illustrations to support your good behavior.
One is that of the late Jackie Kennedy whose father Jack Bouvier was not wanted by her mother at her wedding. Jackie insisted and while Mr. Bouvier was too drunk to attend Jackies mother, Janet Auchincloss was seen as gracious and extremely loving for allowing "nothing to spoil" her daughters big day.
The second illustration comes from a young woman I counseled many years ago. Her father had not been in her life for 20 years and when she married she had recently reconciled with him. Her mother, so distraught and clearly just so selfish, threatened suicide if her daughter went ahead with her plans. My poor client was two weeks away from her wedding, miserable and begging her fiance to elope. Naturally she loved both her parents and had removed herself from their disputes because she was one very brave and mature young lady. She came to see me immobilized, distraught and in a great deal of pain on what should have been the best time in her young life. At the end of the session she felt better and she went home, called her mother and said:
Look Mom, I would hate it if you did try to commit suicide and I would really miss you if you were not at my wedding, but I am going ahead with my plans as they are and thats the way it is going to be. Her mother did come around and she did not misbehave. Years later when she was many years out of therapy that young bride called me after her mother had died and in one of their last talks together, she told her mother that she forgave her for being such a jerk to her before her wedding. Her mother, dying, remarked, you are right, it was pretty jerky of me to let my past hurts interfere with your happiness.
Thank you for not doing this very same thing.
Thank you being loving your child unconditionally and being selfless enough to put her needs and wishes first.
You set the bar now let the rest of the family follow. If they meet the standard you are setting so be it, and if not, well at least you have a child who will be parented by both her parents as she deserves.
I admire your class and selfless love for your child and I appreciate it that I did not have to tell you to behave better as clearly you had already chosen the high road on this.
Have a great time. Dance up a storm and celebrate.
You are a parent many could learn from.