Does she think some men are just born either circumcised or not?

Is it that big a deal? It's not like she's asking you to actually visualize his full-grown penis. All males are either circumcised or not, it's simply a fact. Circumcision is actually a legitimate decision parents have to make one way or another for their baby boys - you might as well get used to the idea now. She's probably trying to give you some helpful advice, since she's had experience with both.

It's something you need to think about. There are in fact differences in care needed between circumcised and uncircumcised. She was probably telling you that so you could ask them both about the matter and get some first hand information. It might have been less embarrassing if she had just sent you a Wikipedia page or other article about it, but she's a mother in law. I think they're more or less obligated to at least make you feel embarrassed.