My MIL lives in her own home on her own. She is 89 and dealing with some cognitive decline. She frequently crawls due to back issues. She is not falling as far as we can tell. My husband took a picture and sent it to me and my sister-in-law. I am a teacher and therefore a mandated reporter. I don't think this is ok. My sister-in-law, who is the medical POA, thinks it's ok due to it not being a result of a fall, but instead because of back issues. She is really entrenched. If we do anything, there is a strong likelihood that the relationship will be damaged beyond repair. She does a lot for her mom. She is the primary caregiver.
So, is it ever ok to let the elderly crawl?
My mother fell 95x while living in Assisted Living & Memory Care. If she were able to 'crawl' to avoid having taken those 95 falls, I would have been very, very happy to have agreed to having her crawl, to be honest with you. So there you see the other side of the coin: were you to know ALL of the gory details of this situation, you may in fact change your mind about MIL crawling around.
When my dear father was wheelchair bound, he would fall out of bed once in a while........and he'd crawl to the bathroom rather than call for help. He'd do that out of embarrassment & in an attempt for independence. Was I 'happy' that he was crawling to the bathroom? Not really. But I also understood why he was doing it, so I didn't make a big deal about it.
So, unless you want to take over the care & management of your MIL, I'd back away from this situation and let your SIL deal with the matter as she sees fit. Unless you see your MIL being openly neglected or obviously suffering in some way, I'd let it go. Don't fix it if it isn't broken.
Best of luck.
But you're right in supposing that eyebrows would shoot up all over the place. Why is MIL crawling? - from where to where, for what purpose, involving what risks, and isn't there surely a better way of supporting her mobility?