How would you respond?

So FH's cousin is his best man, but FMIL doesn't want him to be our witness for the marriage license, she wants FH's brother-in-law to sing the license. She straight out said it like this. I'm a little irked by this because, one, why should anyone tell us who should be our witnesses for our marriage license and two, FH chose his cousin to be his best man for a reason not his brother-in-law. I don't understand why FMIL wants FH's brother-in-law to sign our license. She expects him to get up during the ceremony and sign the license and just have FH's best man standing there on the side and then he goes back to his seat in the pew.

FH didn't even bother to say no the best man should sign; he just gave in and doesn't see it as important. I guess I'm more annoyed by it all because FMIL is yet again trying to control things that she has nothing to do. First, she tried to control what I am walking down the aisle to, then what our first dance should be. I really don't know why she should have a say in who our witness is. It just bugs me that she gets what she wants all the time even when she should just stand on the side and keep her mouth shut. And this always seems to happen when I'm getting along with her and everything is fine.

Of course, it always happens that way!! You get along great, and then, BAM something hits the fan....

I think you need to talk to FH and somebody needs to either tell FMIL how it's going to be, or not say anything at all. The best man does this... End of story (especially since that's your take on it...)

Shes playing nice so she can hit you with these type of things, Shes going to keep at it till she gets her way with something. This is the best mans duty FH and BM should both have a prob with this.
Also I signed my sisters certif it was done after ceremony with just officiant , BM, Sis, her new hubby and myself not in front of everyone.
Sorry you have to go through this type of stress but Id find a way to nip it in the bud now or deal with it for the rest of the future