What to do about my crazed future mother in law?

I seriously went through the exact same thing a year ago. The only thing you can really do is: PICK YOUR BATTLES. :) It'll actually get worse before it gets better. My MIL is now up our s about the baby we're having this week... UGH!! But seriously, you and your fiancee need to sit her down and explain "This is our wedding. We accept and appreciate any input and ideas you have for our wedding, but in the end, we're going to go with whatever ideas we like best. And if we need anything, we'll definitely ask you." That's what we did, and our wedding turned out PERFECT! Of course, when she got her feelings hurt before our talk, she would cry and make me out to be the bad guy, etc., but after the talk, things were fine. :) Good luck! And don't let her ruin your special day.