Thank you @Genegeniehunt - it is hard being part of the 'sandwich' generation.
My DSis, who I mentioned has just had to say she can't do the one day a week care she used to do for her GC as she has to get an extra days work just to survive, literally went from my DM dying to looking after her new GC (5 months break).
And in a lot of cases there's an expectation of looking after mum and/or dad AND giving a day a week or a Saturday evening looking after the grandchildren.
I was brought up in the 70s. My only living Nan didn't help but she was a good Nan. We loved going to see her - she'd always have home made cakes in stock even if she wasn't expecting you.
My parents (and my aunts and uncles) didn't have an expectation of help but each of them took turns to have Nan at Christmas, we all piled over to hers for her birthday or our birthdays and share the present opening with her, organised a big bash for her 80th. Because she was the head of our family and it went without saying that we looked after her no matter what.
Even now we still speak about our Nan with a lot of love (and quite a few giggles as she was very outspoken), but none of us were baby sat by her. As we grew up we'd often pop round to see her only to find a cousin or auntie/uncle had also 'just popped in' so there were no grudges that I can remember and I don't ever recall my parents moaning about the lack of help.
Maybe we were odd as a family or it was just a different time but I never remember a 'tit for tat' thing going on. I suppose, though, Nan was always happy to have a visit, always ready with things to talk about and a cuddle so maybe the lack of picking up from school, babysitting or having us for a weekend just wasn't that important.