I'm beginning to suspect there is a super secret Future Mother In Law handbook in existence and once they find out their son is engaged it's mailed off to the new recruit.
I first began to suspect of such a thing when I realized that mine is stalling on getting me a guest list. For my mother I just randomly mentioned that we'd be wanting a number and in a week she mailed me a fun random doodling of names, addresses, and their favorite condiment. As for the other side, we had to drag a number out of her so we could order invitations. Still waiting on a list of names and address so we could send the invitations to someone instead of dumping them down the sewer (sadly we don't have enough passes for the sewer mutants).
This is one of the best ways to stall the wedding (assuming they aren't paying for anything anyway) without causing any major structural damage.
There must be a lot more to the FMIL Handbook on how to really stop a wedding you don't want to have to go to. These can include driving the bride nuts by constantly changing her mind, refusing to have anything to do with it, or saying she really wants to help and then when the bride does ask for help feign not hearing her.
I'm onto your secret MIL society. You might be more powerful than the freemasons, more reaching than the Templars, and more secret than the Justice League but I will find you and end your torture of brides and future brides everywhere.
Or just to help hold back the tide if you can't take down the MIL organization, you could always grease the wheels with a really nice gift.