My advice is "GO". Nothing says that if you get 1 hour, 2 hours, 3 hours away from home, and find it's too uncomfortable that you can't just turn the car around and go home. At least by starting out you know you tried. You just never know where you'll go unless you try.
Things you might do though are: Have your partner place your suitcase in the car for you, and ask the hotel staff, or your sister to take it out. Put anything essential in a small tote that you can lift easily. Call the hotel in advance and let them know you have a disability, and can not use the stairs, and make sure they will lift your suitcase out, and deliver it to your room if needed. I always call ahead and ask for a room with a fridge, and microwave. Your partner, and sister can also help with a cooler of ice, so you can make cold packs if your back needs it. Bring along a few pillows for the drive, and get out for a wal****asionally. If your seat is adjustable, try moving it to different positions occasionally. If you have time, you might want to order one of those car seat heater/massagers on-line. They run around $25. Stop by the library and get a few books on CD to distract yourself from pain if you have any.
No one can see the future to know how you will feel on the day of the trip, but try not to make self limiting decisions based on what might happen. We WLS people know better than most how wonderful life is when we explore options and take chances.
I hope you have a great trip, and that you and your sister have a fantastic time together!
Kat