Same problem here Cl with MIL. Have had no luck in saying things like "Oh look - theres a bit of something on your cardi, shall I get a clean one out?" which gets met with "It's only my house cleaning clothes - nobody is going to see me...."
What is happening I think is that she can't use the washing machine any more, so she is washing bits she notices off under the tap, and that then becomes clean to her.
When her daughter comes to stay, we can distract her and get a load of stuff in the machine and restock the wardrobe with clean stuff.
So...(I live about 15 mins away by car and am there about every other day)....I have to be sneaky.
If she steps outside - "It's cold out there - put this thicker jumper on"....(Stash disgusting cardi and take it home to wash)
If we take her out for lunch -"We've got a table by the fire - you'll be to hot in that - put this on....' (STASH, wash.....) I'm sure you get the gist.
MIL does all her own washing, ironing, cooking finances apparently- and when we leave her she often says -" do call in when you're passing by." This could be in the evening after OH has been called first thing in the morning by the carers saying that they can't get into the house - to find her sitting on the floor by the bed unable to get up..... She didn't fall - she was just "doing it for fun" No awareness of her impairment whatsoever - not her fault possibly, but it makes it so terribly difficult to try an look after her.
It's basically, don't ask, just do it quick without a fuss, ignoring umbrage. It sort of works for me.......I just worry about the undies......
Subterfuge, white lies and a bit of light pilfering is the answer. And a very large hand bag.