The only I don't like about my mother in law is not her fault - she is in the early stage of dementia / Alzheimers and her short term memory has left the building. This mostly manifests its self in her putting something down (like her keys, wallet (with various cards in it)) and not being able to remember where she put them. We've had to replace her driver's license 3 times in the 18 months we've been here, and I've lost count on her bank ATM card.
Hell, my wife loses her keys, wallet, glasses on a daily basis. I used to get all worked up and would search the house over. Now I just sit down and watch TV while I'm waiting for her to get ready. (This in no way is meant to make light of your MIL's condition)
No apologies necessary - it is what it is. Maybe saying her short term memory has left the building is a bit too much - but it certainly is not as good as it used to be. And she does realize it.
I've found that it is usually best to just let her go on - sometimes she finds what she's looking for, and sometimes not. We've tried using the Tiles - she kept taking them off her wallet (which is where she really needs it).
We do end up telling her several times plans we have, to make sure they have a chance of sticking.