Hi Allison, i am sixty three so not far from your sisters age so i would feel her concerns and fears!!
you will love cruising it seems the way to go if you are handicapped, as everything is on the ship for you and some.
We were able to get off as they help you on to the tenders, actually lift your wheelchairs is your sister able to walk some if not you should look into what the ship has to offers for handicapped people!
I will be glad to answer you questions if i can.
I have M.S. so pretty much confined to my chair.
which islands are going to?
Any questions just ask. Hester