I don't see how Susan can not attend her brother's wedding without explaining to her parents exactly why she isn't going. We had another thread recently where a brother was contemplating not attending his brother's wedding and a lot of the advice centered on what message not attending a sibling's major life event would send to the rest of the family, and to the groom.
And once Susan has told one or two people, it's going to be very, very difficult to keep this a secret. A sister not attending her brother's wedding? People are going to ask questions. A sister suddenly not a member of the wedding party? People are going to ask questions. And even worse, if no explanation is given, people are going to gossip and speculate and come up with their own ideas about what happened.
And it is an issue that needs to be dealt with. It's not just about the wedding, as PPs have mentioned. It's about every single family gathering from now on.
Susan is a person. She is not decoration. Her looks don't matter. Treating her like a human being matters.
I cannot imagine the pain that the bride's demands have caused. To go from the joy of being in your brother's wedding party and celebrating with him, to discovering that your FSIL cannot stand the sight of you--that's soul crushing.
As for whether or not someone would do something like this, yes, they can and they do. My nephew uses a power wheelchair because he has a spinal cord injury and cannot walk. He's been like this since birth. When his cousin got married, she wanted Nephew to be the ring bearer, because she knew that he would take the responsibility seriously and she wouldn't have to worry about the rings. She also wanted his younger brother and sister to be "flower people."
The mother of the groom was very, very upset about the 9 year old kid in a wheelchair. It would look so strange to have a wheelchair come down the aisle. People would talk. What would the pictures look like? She was not happy to learn he would be in the pictures even if he was not part of the wedding party. Did he even have to be invited to the wedding? Couldn't he stay home with his home nurse?
Fortunately, the bride and groom were in agreement on the matter. They listened to the MOG. Then they told her she was welcome to attend the wedding or not, as she choose, but Nephew was the ring bearer.
The MOG spent the reception "apologizing" to the guests for her new DIL's choice of attendants (she never came right out and said "the crippled kid" the way she did before the wedding). Most of the guests were just confused by what she said.
Nephew spent the reception dancing with every single girl there and some of the married ones, and charming quite a few guests out of their serving of wedding cake.