When I was a young boy, we brough my maternal grandmother to live with us while she faced her last months of life as a result of cancer. In those days there was little which could be done. She was a great blessing to us all her life. I still remember those days so well, so I know a little of what you're going through. That grandmother was one of the great loves of my life. I am looking forward to seeing her again in a place where there is no more pain, and no more parting. It really will be wonderful when we all get together again.
In the meantime, I hope and pray that you and your sister have a great many more loving years together. Yes, miracles do happen. But if God allows another outcome, then it's only a matter of time until we'll have an eternity together with those of ours who love our God. What hope we have because of his love.
I wish all of you the best - and the best is nothing less than the blessings of our Lord. He is faithful, even in these times.