S's sense of herself - she still has one - includes, very firmly, a belief that she should be and is helpful to others. This is absolutely correct, she was a single mother from the time her kids were very young and a teacher whose roles were often specifically and always predominantly to do with caring for and helping those in difficulty of one sort or another. I believe she would like to still be doing this sort of thing and is frustrated that this is just one of the things that she can no longer do - or as she sees it - is not allowed to do. I have realised this from listening to numerous versions of a conversation where she is trying to sort out girls' problems (she finished her career, premturely, in a girls' school).
Her daughter and I have found that a sure way to get her co-operation, even when she's angry, is to ask her to help us.
I would love to find some other way(s) that she can be truly helpful and that we can keep praising her for. The cats we have acquired are one possibility. I agreed to the pressure from her daughters as I believe they could help to occupy and calm her but my line will be that by paying attention to them and stroking them she will be helping them - that will be her vital contribution to their care whilst I'll do the easy things like feeding them (though, of course I'll stroke them a bit too!).
If anyone else has any ideas - it's very hard - please let me know by commenting.