On the comfort front, I don’t know this exact vehicle model, but do know you have a bit of control over situation. Assume you and sis are sharing driving, each a few hours at a time, so you have to pack car in a way that you have good legroom in either front seat. The passenger seat should be able to recline at least a bit. Sleeping while “off duty” can extend your driving day by several hours.
(An alternate would be to have the “resting seat” be behind the driver? Sis is petite so if she is driving she is slid forward, giving you leg room behind. And if you are driving, seat is slid back and she does not need much room? An idea…) Also simply pack up lightest items that take up space and ship them rather than carry these in car. Packing car interior full means limited visibility, so plan on right-lane driving under good conditions, daylight. Good luck, enjoy the time and the ride!
How’s the trip? It sounds uncomfortable but it would help to rest and stretch once in a while.
This is a 6 month old thread so the trip is probably over and they never returned to give an update.