I would not do this to my children. So you need to discuss it with your parents when they get back. It is up to them to approach your Granma.
Despite what Granma says, if they explain to her that it is in her interest for all parties to remain physically and mentally well, so that you all can continue to look after her, perhaps she would shape up.
The only way I could get my MIL ( who had brokenher hip) to come and stay with us for three months, was to play on her love for her son. The 250 mile round trip, on top of a full time job was killing him.
She said, ' I can manage ' and I countered by saying that ' unfortunately your dear son cannot manage. He is suffering !!! I then had to endure her for three months.
My Dad went for respite care which is allowed on the NHS ( I think it was one week). Also, if your Gran was married to anyone in the forces, St Dunstans offer a week.
Other than that you HAVE TO LEAVE them to get on with it and try not to feel guilty. If they turn down offers of help then that is their responsibility.
We are in the midst of it now, ( sighs) and not one of our oldies have made any provision for their old age. They just stagger from one crisis to another and we then pick up the pieces.
We don't let our children get involved though. They love their GP and gist them but I have done the bottom cleaning and bathing etc. ( lovely)
It is very very stressful, and to those who are criticising the post for her efforts, I think she is a Saint and I know how she feels.
Unless YOU have looked after elderly difficult relatives for years, then you can bog off.