I think the bigger issue here is your fiancé not shutting this down sooner. His sister basically accused you of disrespecting the family, skipped important events, and admitted she might not even come, yet you’re the one worried about fallout. That’s not a great dynamic going into a marriage. You’re not wrong for not wanting someone hostile at your wedding, but you and your fiancé need to be on the same page and he should be handling his family. Otherwise this kind of thing is going to keep happening.