You learn normal behaviour from your family (mostly your parents). The current crop of younger adults includes a substantial part coming from broken households, brought up by awful selfish obeast. They reach certain age, shit like this goes down, and they feel somewhat uncomfortable but can't really tell whether it's normal or not because they've seen worse while they were growing up. Hence, they turn to strangers on the internet. You and I would nope the fuck out of that without batting an eyelash. Linked OP was probably cleaning between the folds of her own mother when she was five and all she's confused about is whether doing this with the MIL is okay or if there's some sort of technicality she can use to get out of it.