I'm going to make fun of my sister in law for acting like she's from nyc
Why would you make fun of your sister in law?
She acts like she's from nyc but she isn't. I can't stand her attitude!
It's understandable to expect everyone to act like themselves, but its rude to ridicule others for how they want to be seen. They may be happier acting as if they're someone else and  if you call them out on it, it could cause issues within the family.
But it's so annoying because she isn't from the city, she's from upstate.
I think it would be better if you just ignored her, because what she's doing isn't really hurting anyone. Making fun of her is only going to cause tension in your family, which I think everyone would rather avoid.