I saw my mother in law (MIL) naked within a few years of meeting her. And not because one of us was in the hospital or anything.
You see in Japan, the hot springs baths are naked. Of course they are separated by gender, but you still have to get naked.
It was a little strange the first time I ever went to one (not with my MIL then.) I was in rural, Northern Japan. You should have seen how the old ladies gawked when the big, huge, tattooed foreigner came in! And I mean unabashed, open, discussion included gawking.
It was interesting for me, as well. It was the first time I had seen nipples that truly looked like pencil erasers. It was also the first time I had seen a broad range of naked women from puberty on up through grannyhood. It was quite a fascinating sight to compare all those hips and boobs and legs.
Anyway, what a strange position to find yourself in, naked with your MIL. I am sure she is having the uncomfortable thoughts along the line of “so this is what my son is into” and I was having the thoughts along the line of “so this is where her son came from.”