If you have had a wonderful MIL then you are better equipped to be one I think.
My mother has been truly horrendous to my brother's wife but sweetness and 'old lady' light to my sister's H.
But then she's a shit to my brother too.
She's also control freakery and criticial when it comes to my sister's children, so my sister's H actually loathes her and just tolerates her 'sweet old lady' crap while actually he wouldn't piss on her if she was on fire.
I mean families. ffs.
My own MIL is a toxic critical snob and a shagger. But then thankfully she lives in another country.
Maybe that makes her one in a million too?