Quoting Mrs. Weasley [tech probz]:" My wedding is this Saturday and my SO's sister is a bridesmaid. In the past week she has: Stolen from ... [snip!] ... So should I comfront her, kick her out of the wedding or other? I know I sound petty but she is ruining this whole thing!"
I'd confront her. But if it is going to completely ruin your special day, then kick her ass out.
You will be the "bigger person" and she will make her self look like a fool if she acts up, especially after you give her this chance.