Many thanks for all the advice and support here. I am not particularly close to this ex sister-in-law. Unfortunately, when her marriage to my brother broke down (entirely his fault), she behaved badly in that she stopped all communication between my parents and their g.children. Broke their hearts. I can still remember seeing the notice of the eldest girls wedding in a newspaper, and taking my parents to sit in the public gallery of the synagogue to watch their eldest g.child walk down the aisle.
They were not invited to the g.sons barmitzvah - (resulting some years later of me persuading my eldest boy to become barmitzvah - strange in a strongly atheistic family). He understood and his g.parents were so very happy.
However as soon as my niece was married she started to visit her g.parents and I have had a strong relationship with her ever since. I have always been invited to all their family occasions and they to ours.
I have seen her Mother over past years at my twice yearly visits to my niece, and we have got on pretty well. The problems were all many decades ago.
So when my niece (now, herself a g.parent) asked me to telephone her Mum in the home, I happily agreed to do so. The first time, was no problem and we chatted quite well. Does seem to have been serious deteriorating now - perhaps Lockdown is part of the problem. She did know who I was, although (I heard today), that when one of her gdaughters phoned her in the afternoon, she did say she had received a telephone call from someone that morning, but could not remember from whom.
I now understand far more about this condition, and also know not to try to correct anything as it could lead to more confusion and stress. Just seemed so strange to me when she told me my brother was not well - I know she was referring to her second husband of some forty years - but it was still a jolt to me as my brother has been dead for some 15 years.
Thanks to everyone, do love how we can so quickly get some good advice on virtually any subject on Gnet.