She's in her mid 90's and in an assisted living facility. She's obessed with things missing (stolen then returned sometimes) ..now tonight she said she got sick at dinner and 'they put something in my food'..I just don't know how to deal with this and I find I don't say the right things and she gets upset with me because I don't believe her. I think she has dimentia tho' no official diagnosis but has been on meds for a couple of years..doctor hasn't officially said it. Don't know how to respond when she makes these accusations. I'm told there's no way to change her mind that she believes what she believes and no amount of talking can convince her otherwise..
If indeed your MIL has dementia then I agree it it pointless to argue with her. But it is also very damaging not to defend your son.
I think you need to make other arrangements for her.
People with dementia live in their own worlds. It is generally more successful to enter into their world than to expect them to come into ours. Misplacing things or even hiding them and then forgetting that and accusing someone of stealing them is VERY common in some kinds of dementia. For that matter, having things stolen in a facility is not unheard of, either. Another resident may simply think it belongs to them, or may have lost the concept of ownership entirely. Trying to convince Mom that the item hasn't been stolen is pointless. The goal should be to comfort her and make her feel valued. "Oh Mom, that is really too bad your nice hairbrush is missing. I know how much you like it. Let me try looking for it for you, just in case it got misplaced or someone took it, realized their mistake, and brought it back." Do a thorough search. If you find it, be glad. Resist saying "I told you so." If you can't find it and it something that can be easily replaced, just replace it.
You may discover that Mom has a few favorite hiding places, and once you know them it isn't so hard to find missing items.
"Oh my goodness! I am so sorry you got sick at dinner. Are you feeling all right now? Can I get you some ginger ale?" Try to avoid commenting on the "someone put something in my food" accusation.
And remember, just as your mother may have "undiagnosed" dementia, so may other residents. Sometimes when she says someone took something or was mean to her, she may be absolutely right. (And she may sometimes have something that does not belong to her.) Don't tell her the accusations are wrong, but try to get beyond them to find or replace the item or to make her feel that you take her seriously and care about her.
A book you may find useful is "Creating Moments of Joy," by Jolene Brackey.
And don't feel bad if you don't always come up with the "right" answer ... sometimes there is no right answer and we just stumble through this as best we can.