What an awfully sad story, so young and so much to live for!
What you are about to go through will not compare to anything you have ever experienced but you will cope and you will be strong for your 'sister' and provide her with your love and admiration. She will teach you things too in these next months that will be invaluable to you.
My husband passed away 2 weeks ago, he had brain cancer, it's an awful one to have, it takes so much function away. To watch a man, so bright and so capable to be reduced to baby like beginnings was the hardest thing I could ever imagine experiencing. But, he showed me how to die. He taught me how to find peace and how to face that moment with strength and courage.
We all die. Not all us know how to when that time comes and these experiences are oh so special. I'm still in the depths of my grief but even still I know how lucky I was to be with him all the way to the end. The lessons he taught me along the way and how now I will live better for having had him in my life.
Stay strong, keep loving her and hold hr when she needs to be held. Cherish every bloody sec with her.
Good luck, you're both in my thoughts.
Jo x