Quote Originally Posted by o_gal View Post
I was going to post this - find 2 or 3 friends and enlist their help. Give them a picture of her, and have DH point her out to them prior to the ceremony. If she causes any trouble at all, even the slightest bit, they have your permission to confront her and remove her, if necessary. If you're lucky, she won't cause any trouble. If you're mostly lucky, a single confrontation from them will cause her to behave. But warn her that she will be removed (emphasis on "will", not "may be") and everything will go on without her.

And yes, she's responsible for her sister, 100%.

It might be beneficial to introduce these friends to MIL and AIL at the start so they know you aren't bluffing.