Sorry for the delay, I just recently heard from Susan.
The wedding will go ahead as planned, however since Susan has two young children to keep track of, it would be easier for her not to be in the wedding party. This is what Susan plans to tell anyone who asks, as Susan told Louise she would keep this "disgraceful incident*" between them. There was someone sort of confrontation between them that I don't have details on, but Susan seems pleased with the outcome.
Susan did tell me that everyone has at least one really stupid moment in their lives, that hopefully this was Louise's, and she will let it drop provided nothing like this ever happens again. She isn't going to hold it against her brother for marrying Louise. They love each other and make each other happy. And Susan loves her brother. She really wants his weddding day to be happy and unclouded, and she's willing to do her part to make that happen.
*Actual phrase used when Susan spoke to Louise.
PS-to answer recent queries, I don't think there is a genetic componant to Susan's arm. Both her kids have all limbs and she wasn't particularly worried about it when she was pregnant with them.