I am so sorry for the challenging news!
I understand your concerns for travel. There were times on trips that I literally thought I could just jump out of the car. Thank God I didn't, cause that would have hurt soooo much worse. I really need to be able to lay down when I am traveling. If, that is not possible, then recline with your feet elevated. Also, I always pack plenty a pillows, the bumps and jabs can leaving you feeling beaten by time you arrive. When you have a chance to stop, get out and move around a bit. Gentle stretching will help. Select relaxing music (don't put the driver to sleep though) to keep you from being anxious. Keep meds in a convienent, safe place.
I agree with keeping the focus of your sisters condition in the most positive light possible! I will too! One of incredible strength, continual healing, and amazing resolve.
Image her healthy,peaceful, active and joyful. All my best to you my friend. Love to you and sis ..Di