When did it become etiquette accepted to tell everyone when someone did something mean to you and try to get them to hat the person as well? Trying to get everyone to cancel, essentially ruining the bride and groom's wedding is not proper etiquette, it's revenge plane and simple. Hurt feelings should be addressed to the bride and groom, these people are adults, they don't need to go running to their parents.
As for the husband to be, he is being asked to pick sides between his soon to be wife and his sister. He can not win, no matter who's side he picks. As my husband would put it, there is a bullet in every chamber.
The sister should not attend the wedding, and if asked why she was not there respond with "The bride and I had a falling out, it's a private family matter" and end it there.
What happened to dignity and grace?