In the end it's your grandmother's decision though I hope whatever happens she's not in pain of course. It's really hard to lose a relative, especially to cancer when you have to watch them suffer, so your family is of course going to have a hard time accepting it. But as long as you stay strong for your grandmother, that'll be for the best.
And not to be off topic but at my college we recently did a topic on cancer and I think the type that kils the most is lung, followed by breast cancer and then prostate. But that might be the statistics for the U.K so I don't know, I just thought I'd share the information.