Oh Poor you, I totally feel for you, My FMIL is exactly the same!! Needless to say I didn't invite her dress shopping!! lol She thinks that were ridiculous for going down south and that its ridiculous that I'm wearing a long dress.....I'm pretty sure she just talks to hear herself!! lol
Don't let her get to you, you guys are going to have to tell her together that this is YOUR wedding and its what You guys want to do, and it really doesn't matter what anybody else wants. Good luck to you!! ((Hugs))
Ps - My mom would have LOST her mind if my FMIL said anything while we were dress shopping, but FMIL doesn't have the balls to say anything anyways