Right, she's clearly more bonkers than I previously thought and looks as tho she will not be moved on this one.
You must now either play dirty or put up with it.
Christ I feel so sorry for you but I so want you to be happy and not have the day ruined by a rat bag of a sister in law!!!
Options...
1) try you best to pick up some germs from somewhere to make her ill before the wedding, I'm thinking a used tissue/ really rank dishcloth. If she gets poorly hopefully she won't come.
2) similar to the above, give her a dose of food poisoning. Extreme I know but this now calls for extreme measures!!
3) speak to her husband on his own, tell him your woes and appeal to him to address the situation.
4) last option, as someone else said, speak to her parents and tell them exactly how THE TWO OF YOU are feeling. Hopefully as I'm imagining they might be a little older they won't be quite so "liberated" as the "whip-it-out-mammary-mammy" and will take your view and either talk some sense into her or - in no uncertain terms - lay "the law" down i.e. some common sense.
She presumably wouldn't want to go upsetting all 4 of you just to prove a point?