If it was my mom, I would simply say, "It's not your penis. It's not your choice." And never engage her on the subject again.
I don't think it's weird to know if your Dad is intact or not. As a child I occasional saw my Dad naked so of course I knew.
I also know about my FIL because DH told me he was "had to be" circumcised a few years ago. He had a colonoscapy, ended up with a perforated bowel, and a few surgeries later he was missing a foot and a half of bowl and a foreskin.
I like to do a lot of skin to skin contact with my babies during out babymoon so my kids have been naked a lot the first week. MIL was holding DS and his blanket came loose and she saw his penis. She asked us when we were going to circumcise. When we acted a little shocked at the question (because she had heard us say it ain't gonna happen many times) DH responded with, "Look at it! You want us to make it smaller?" I just about fell off my chair it was so funny. MIL was not at all amused and said some really ugly things about how disgusting an uncircumcised penis is, etc... It was pretty uncomfortable for a few minuets because FIL was right there.
I'm sure she did not know that I knew about his history. I'm also pretty sure she did not know that FIL had already asked me about circumcision and that he was really happy when I said "No way!" I believe he said, "Good. Don't ever do it."
Now that is knowing some pretty icky things about parents.