Recommendations on a cotton incontinence underwear for elderly parent?

Your mom could be developing pressure sores from the all day sitting or sores from the moisture of not changing often enough and that won't change no matter what product she uses. Get her a good pressure relieving seat cushion and encourage her to use a barrier cream (Tena makes one but any favourite from the baby department will do).

They have many good pads that would be a change. Tena are so soft. Go to Tenas webpage and you can get a sample.

There is a brand, company that makes a very absorbent bed pad that I used for my Husband. They also make underwear as well as liners. The company is Conni.
You might also want to try a good barrier cream. They can be pretty heavy so it is important to “pat” the cream on and not rub it on.
The cream that I found to work well was Desitin I often combined it with another cream that had calamine as a base.
The other thing that is very important since she sits a lot is changing her position. Something as simple as shifting in the chair will help. Getting up is better but I understand the difficulty. The important thing is to relieve the pressure and increase circulation.