It strikes me as far too much at 70, OP. Younger parents often just don’t realise how much more tired most of us get when we’re getting on a bit. I think you do have to tell her, difficult though I know it will be.
Dh and I (sometimes just me) used to do one day a week with the first grandchild - I was 67 when she was born. That was fine, especially since she was still having 2 good naps a day, but when the second arrived only 15 months later, I told dd sorry, but it was going to be too much.
We offered to help with childcare costs instead, but I do realise that we were fortunate to be able to do so.
I once did virtually a whole week with the first at still under 2 - it was a one-off emergency, since the baby was in intensive care with a bad case of bronchiolitis.
I eventually came home feeling like a wet rag, promptly succumbed to a stinking cold, followed immediately by shingles! I still put that down to feeling so utterly wiped out.
A friend of 70 helps on a regular basis (on her own) with twins of around 18 months now, and freely admits that she finds it extremely tiring.