Your MIL might have a UTI if she has had repeated bouts of incontinence. It sounds as if she would benefit from being on a schedule.
When my mother got so she couldn't toilet herself well any longer, we made sure we got up to assist her at night. One night, she kept forgetting that she had gotten up, and she tried to arise 5 times, requiring my going to help her each time, to make sure she would not fall and would wipe herself.
As she became more and more feeble, we planned around getting up two times a night with her. My OH did the early morning time (anywhere between midnight and 2 AM) and then I was "on duty" in case she got up between 2 and 6 AM. We spent about 3 years like that. I have kept my job; my OH is retired.
I can't imagine attempting to leave my mother on her own under the conditions you are describing. Is there a reason why overnight care has not been sought by her family? These are not accidents, as you know. No one likes to soil herself, and if she has dementia such that she doesn't realize what is occurring, then she seems to me to be way past living on her own.
Sorry - I know this is blunt. She needs assistance for more than a UTI. If she has assets, it would made sense to use them to place her in an appropriate care home, where she will be attended to properly.