Try not to be too upset at the anger, you have given her a huge shock, and to her it feels like you are rejecting her and her children. Everyone says things they don't mean when they are angry. You need to decide quickly whether you are going to go or not, not leave her in limbo.
If you have taken that decision to go, and you are sure it is the right thing for you, then in a few days time you need to sit down with her and say you understand her feelings and that you will miss her and her family terribly, but feel you have to do what you need to with the time you have left. Children never accept that their parents are going to die until it is blatantly obvious!
If you are not going to go, then decide that quickly and tell her you have changed your mind because you realise how much you will miss them.
At 10 and 11, in three year's time they will probably be reluctant to see boring old grandparents, and not need you, so if it was me I would hold off for a couple of years, but only you can decide, just do it quickly!