I can feel you bro.... and i think it is very common. It happened to my mom as well, even more obvious after my father's passing. 
But look at generally fact- .... many people have some mental issue ( i am not say crazy that type). Anxiety, depression, schizo, OCD, ADHD there are many type- then demintia, alzemer in older stage All affect many healthy adults male and female.
It is just that -  in elderly female it is more obvious. one of the key reason is that they are housewife, they dont work and their world is small.
their circle is other housewife in the neightbourhood, or their relative of which are like to gossip

These are few common traits.....
> never admit that they are wrong
> always find something to blame ( eg when drop a cup of water, will say the cup is slippery and water is too hot)
> dont appreciate what you give then... like bring to a nice restaurant, got 5-6 dishes is nice, but they want to compare just one dish and say other restaurant is nicer
> or say far, expensive etc... never want to give credit.
> like to say bad things, and gossip about others.
                  If some one got a new house- will say loan only, owe to bank a lot need to  installment many year.
                  if people buy new car- then say must be doing illegal business
                  if people go travel, then say... haiya- so cold go there for what... or japan there got radiation.
> and more ever.... Schadenfreude i “pleasure derived from another person’s misfortune.”
if this happen to you mother... then just act chill. Dont get too emotionally affected especially when you see the above explaination. Let her be, she will life in her own world. It is very hard to explain, and justify with proper logic and expect it to change.
just do our part, thing what is needed will be fine. at time- we cant and wont do more- coz wont be appreciated.

Also to add.... this type of people, will always wanna find a villan.
          - this week is this neighbour, next week is that relative, then it is that shopkeeper.
          - they always see thing in their own perspective. they are always right, and others condition, situation they dont care. but in short, they need a villian to justify  or make her feel better.

u may also see, this week she is good with these few people, then next week, that complain that people go issue, but be good with other group of people. and same things happen, rinse and repeat.
i can only ask and let her say. dont even bother to explain or understand in depth. all got their own perspective.