Susan needs to talk to her brother, her parents, and any other close relatives and air this now. Why? Because if the bride succeeds in booting sis from attending the wedding without any repercussions, she will not stop there.
Dinner with the in-laws? "Ooooh, honey, you know I can't stand seeing your sister's arm...make sure she isn't invited."
Attending a relative's birthday party? "Sorry, unless Susan is wearing her arm I can't possibly be cheerful."
Bro and SIL have a kid? "Sorry, we can't let you near little Morris, seeing your arm would traumatize him. And by the way, you won't be attending any family holiday celebrations with us there either."