Recently a dear girl who 'does' for the aged around my area, who is well known and trusted by all and has worked for one older person for at least twenty years came to me very upset. The women she 'does' for had accused her of moving her things and although not saying she was stealing, she started searching for certain items, like a stool, that she 'knew' she had that was worth some money. Food apparently was missing etc, etc.
This I was told started about two years ago and has steadily worsened to the point that the woman concerned who I should say is 89 had started phoning neighbours to say that that this young woman who herself is about 48 had been taking her things, and had messed up the Christmas presents and had kept some M&S voucher cards that she asked her to buy for gifts.Naturally people found this hard to believe and told the young women concerned hence the visit to me for advice.
It became clear from everything that was said that there was some form of dementia and although it was upsetting I asked her what she wanted to do about it; confront the older woman, stop 'doing' for her, what did she want to do? When she thought about it, she first said she thought she should get her Daughter to come over, although the mother and daughter are estranged, she felt it was not her responsibility to get medical help for someone else mum. She realised that the people in this village knew that she was a reliable and honest person. She also felt she could not abandon this old woman in her hour of need so to speak.
I advised her though to make sure that when she was dealing with money she must keep a record and if possible have a witness to any financial dealings, and frankly missing food and ornaments were clearly things that anyone who knew the home would realise things were still there so not to be concerned.
She now realising that the old women was ill, saying that she often did not remember after a few days what she had accused her of doing, or taking. The main problem was that people might believe what was being said. Once assured that I felt that would not happen, and advised her to go to the neighbours who knew of all this and talk to them as she had with me, and I felt sure that they would reassure her as well that they knew that the old woman was ill and everyone would rally round to assist in anyway in the absence of the daughters lack of interest, and last I heard this is what is happening.
I hope this helps, every set of circumstances is different, the secret I think is to remember that although you MIL is a normally difficult person, that there is something else going on. My advice for what it is worth is don't take it personally, and don't rise to things she says, it will make you ill, and cause tension between you and your family. Just make it known that you believe she is ill and try to get her to the GP for as much help as is available, which seems very little sadly. But if it is vascular dementia there is some help available I believe.
Good luck with this, these situations are very hard to deal with in families but you will get through it, just keep that emotional distance to protect yourself but you can see that the parent is becoming the child and sad to say that is how you are going to have to view her behaviour.
Best wishes for the future.