Oh, don'cha just love it!!! You know:
"Stupid people don't know they're stupid."
They think they are smart.
I had a complete stranger argue that my known-to-be-purebred Yorkie was a Silky. Someone else "has a friend who breeds Silkies and they sent her the picture of my baby, who said they could tell from the cellphone camera photo that it was a Silky." Well, all righty then...that settles it...case closed...the former owners, and purchasers of my dog, clearly know nothing about him...!
In the case of a MIL...future or existing...what you do about it depends on so much.
Speaking of MIL comments...oh, and I've got lots of 'funny' [odd] comments in my storybook...my former MIL said I was "feeding my [human] baby too fast," my at-the-time-DH-and-children's-father echo'd his umbilical-attached-mom's sentiments and further embellished with a lie that he'd "talked to a lady doctor at Boston Childrens' and she said the same thing"...but OMG!...you should have seen the look OUR pediatrician gave me when I told HIM! TOO FUNNY! He said, "if he's got his mouth empty [which he did] and open [which he did], waiting for the next bite [which he was], you are feeding him TOO SLOW!"
If MIL doesn't believe he's a Yorkie, even though your papers say otherwise, maybe you could tell her you found out he's not...that he's a Great Dane...or a St. Bernard...or a Sheep Dog...because someone YOU know/work with/just met AGREED WITH HER and said so!