How do you have an annoying soon to be mother-in law to relax?
After your married move to another state. Then change your phone number!
Chill pills or drugs
Keep her supplied in her "beverage" of choice.
Weddings are a tense time for most people. Hopefully she will relax after she is your mother-in-law. Remember you are about to marry her baby. Good luck!!!
There's no way you really can. My mother-in-law's great, but two days before my wife and I got married she started flipping out. Just ignore her as best as you can.