I don't care for an in-law, but as a man caring for his mother this was the first huge taboo hurdle I had to face. These days I wouldn't know a taboo if it danced out of the airing cupboard wearing a top hat and doing jazz hands; we're all just bags of flesh and bone, and dementia doesn't care about our social airs and graces. So that side of life has been shelved... hopefully in the same way that professional carers and nurses manage it, not in the way Norman Bates managed it!
I do worry sometimes though.
Anyway, I realised that if I didn't deal with Mum's hygiene issues -- no matter how potentially awkward or disruptive -- nobody else was going to, and Mum was the only one who'd suffer. She was starting to get a lot of UTIs and that couldn't go on. So I became an expert at scouting round her home for underwear and replacing it with new/fresh pairs. I used to worry that it had to match exactly, but Mum couldn't really tell the difference. I'm not going to say it was easy or without problems, and i didn't have anyone else I could call in to distract Mum while I was searching, but it's surprising what you can get good at if you have to.
Or maybe I'm just a borderline psychopath... dementia makes you examine yourself just as much as the people we care for! I guess the time to start worrying is when you stop examining yourself, but by then it's too late! However this weirdo self-analysis isn't what you came here for, is it?
To be a bit more helpful, you could try doing what I did initially... I just bought a ridiculous number of pairs of pants and left them where Mum would find and hopefully use them. On a statistical basis, the more pants there were in the house, the more likely she was to find a clean pair when required.
Of course the main problem was soon that she wasn't changing her underwear at all, no matter what state they were in. That was when I really had to take a deep breath (no an ideal phrase under the circumstances) and get even more involved than I ever thought I'd have to be in anyone's life, let alone my mother's. But needs must.
The only good news I can give you is that the more our barriers got broken down, the healthier my mother became and she hasn't had a UTI for over 18 months since we became, er... bathroom buddies. Dementia take you to some strange places (personally, I'd have preferred the Lake District). But on the bright side, I can now sing Old MacDonald Had a Farm while wielding a wet wipe, which is a skill I never thought I'd have. I fully expect to be rewarded with an OBweE for services to light entertainment before my days are done.
In the mean time I'll settle for occasionally posting shameless waffle like this in the hope that it's useful for others starting on the same slippery slope... if only to emphasise how important it is to get carers involved ASAP to help with the awkward parts of this journey. I was just never good at asking for help... a bad habit I got from my mother, and a very bad habit indeed for a carer! But as bad habits go, it's probably better than picking your nose.
Good luck with your MIL.
PS OK, so I pick my nose too.