If distraction doesn't work, you could try telling her you'll take her tomorrow.
I often said this to my mother when she was wanting to go home to her long-dead parents. We couldn't go today because the roads were very busy/icy/the main road was closed because of a bad accident/my car was being serviced, you name it, I had any number of fib-reasons. But they always pacified her for the moment and she never remembered that I'd said much the same before. If your mum's short term memory is also very bad you would very likely find the same.
Whatever, it is most definitely not your fault - it's the beastly dementia.