But.. history is important, I think, and the actual comments they are making. Do you/your mom have a good relationship with them otherwise? Did they treat your mom and her siblings differently or with contempt growing up? If so then yes, they're nasty and would be getting a complete cut from my life.
I guess I can't think off the top of my head of a comment in this situation using "real children/grandchildren" that would have good intentions, but there could be some. I've had several people that I know love us and our boys, and support our adoption 150%.. ask things about the boys' "real parents". To a lot of people, biological = real. Not more important or loved or permanent or whatever than the adoptive relatives, they just don't know any other way to term it.
The main thing that you need to remember is that your grandparents ARE your grandparents, and they loved your mother and you as their child/grandchild, no matter what anyone in the world says.