You really need to look into renting a wheelchair or
scooter for your mother. That is the most crowded time of year, and she will find long lines things other than a few attractions. Here is a list of things that the GAC does NOT work for:
restrooms (20+ minute lines are common at xmas)
restaurants
character line
shows (have to wait for the next show)
parades
fireworks
street shows
If she is not a "ride person" then she will likely spend most of her time doing the above...
Also, she should not expect to be able to sit down on the park benches reliably, as those tend to be crowded at non-peak times. Also during peak times like xmas, some counter service restaurants will not allow you to sit unless you are eating, so she should not count on sitting there either...
Also, for stamina or mobility concerns, disney's policy is to rent a wheelchair or
ECV. GACs are handled differently depending on the stamp, the time of year, the time of day, how many people with disabilities are ahead of you, the CM, and other variable factors. While your nephew may have had one experience, it is likely yours will be different. At such a busy time of year, I would guess it to be unlikely that you would receive the same accommodation, or if you did you would not receive it consistently.
Also be aware that the GAC will often make lines LONGER, as they can only allow so many people with disabilities onto the ride at once - for instance, only 3 are allowed on Buzz Lightyear, and if there are already 3 on and there are 5 people ahead of you, you will have to wait for the ride to cycle completely at least twice before you get on (and this is AFTER waiting in the main/FP line). Last time I was there, I waited 45 minutes for Jungle Cruise when the standby time was 5-10 minutes.
I am not trying to be rude or anything, but I want you to be prepared for how crowded this trip will be, and how a wheelchair or scooter will likely be a better choice. I use a wheelchair in daily life, and I can honestly say I enjoy the parks now even more than I did when I was able to walk!
(btw, people walk between 6 and 9 miles a day in disney - a good fact to share with your mom)