This thread covers exactly where I am at the moment. Both my Grandfathers "died in harness" before I was born. One Granny had a spinster daughter who lived with her, the other a son, both DGMs died suddenly in their mid 70s, so my DM and DF were never burdened with caring responsibilities.
Sadly for me this was not to be the case. My own DM suffered from anxiety and depression and was always 'ill' or unable to cope, demanding help from her DDs, particularly me as she and DF moved to a flat just round the corner.
My DH was an only child. His DM was acknowledged by all as a 'nightmare' and she showed animosity towards me from the day we met. Nevertheless, once FIL was diagnosed with Dementia, and admitted to a mental health hospital, she demanded that she should come to live with us. Thankfully DH said no way. However she would not visit FIL (too upsetting) so I became his only visitor because I was already visiting my maiden aunt who was in the same facility.
So, there I was mid 50's. having an awful menopause, working 2 jobs (as was DH) caring for my DM, MIL, DH's Maiden Aunt (housebound after she broke her hip) sharing responsibilities for my Maiden Aunt with my sister and trying to fit in visits to my DDs - both then at University. At one point 4 of them were in different hospitals simultaneously and I could not win because each expected me to be there at every visiting time.
Then when we had a year when all five of them died' A few years of respite then my DF died, I was made redundant, had a 'breakdown and my sister (whose husband was dying of Cancer) and I become responsible for my Dad's brother, who had developed dementia & eventually had to be sectioned as he refused to move into a local care home, where, in fact he flourished and had a good few years.
Then it was my DH who began to show symptoms of Dementia, cared for at home by me for 4 years until it all became too much for me, then a move to the same care home until he died last September.
Because of all this my DDs are well aware that it is a move to the same Home as soon as I become unable to function independently. They already do so much for me, when I tell them it is too much they insist that it is their pleasure to be useful and I should accept their help with good grace.
Sorry for the long diatribe, but I have recorded it in full to illustrate how chance can lay an impossible burden on one pair of shoulders.