IF they are concerned about getting on or off the ship at embarkation and disembarkation, you can request wheelchair assistance for their medical conditions. They will bring a chair and wheel them to their cabins, or off the ship, so they wont have to stand in line. You just call Princess, or your travel agent, and tell them that this assistance is needed. As someone else mentioned, for any ports that you are just getting off and walking around, just wait an hour or so after docking, and just waltz right off............many people want to get off IMMEDIATELY for shore excursions or just because they are in a big hurry........if you wait, there will be no lines. If you are booked on a Princess tour, you will be escorted off without a long wait. If you book your own tours, you will have to wait until the Princess tours get off first.
If your Mom or her sister need more than minimum assistance, you may want to make sure you have a handicapped cabin, and rent a scooter or wheelchair for use on the ship. Most scooters, or rigid chairs, will not fit through the doors of a regular cabin. Folding wheelchairs will fit through the door. It is very tiring to push a regular wheelchair over miles of carpeting. If your family doesn't need that, perhaps they would like to take a walker, like a Hugo, that is also a seat that they could sit on when they become tired.
Hope you all have a great cruise.