I had a similar issue , though slightly different circumstances , with my youngest son and his wife.
When my grandson was born 12 years ago, I was roughly your age, going through a very distressing and difficult divorce, working part-time, doing school runs for my daughter’s children etc etc. My son asked me if I would look after the baby at least one day a week, because my d.i.l. wanted to return to work. I just could not help them. I only had one free day for catching up with my chores and I was not willing to give that up.
So, I sat down with them and explained it all. I offered to do baby-sitting, if they wanted to go out for the evening. I don’t think they were too happy, but that was all I could offer. I also felt that they were financially well-off and could afford to employ a child-minder or whatever..
To cut a long story short, they understood,
My d.I.l. did not return to work. Our relationship has not been affected. I babysit when they go out for a meal,
I have retired now and everything is hunky-dory. Just last week I stayed overnight with the boys (they have two now), to allow them to go out for a meal with friends .
You have to be straight with them. Your health is your priority.
I am shocked that the rest of the family are not sympathetic to your situation.
I hope things improve 