as someone who wear too, I find that it's hardwe to change a person in a laege brief.

I used ot be heavier when I was bedwridden .  I filled the whole seat with mess.  I now can walk but still have no control.

By correct fit what do You mean?  How large is she? Can She walk?  Does she have problems with leaks?  I'm a 50 year old disabled lady.  I have had problems with incontinence for 25 years. At this point I do need and use adult diapers for complete incontinence. I'm 5ft1in and about 130 pounds which is a bit heavy. We use the Tranquility briefs and boosters. And also Attends.  Other better briefs Secure x plus .  I have found the wider briefs are better to prevent leaks. I don't have much feeling down there. So comfort may mean something different for Me. 

Depending on size, you may try bariatric briefs. They maintain the Large/X Large brief size but add more stretch in a belt like form.

In my experience as a 24/7 wearer of tabbed briefs for both forms of incontinence, Tranquility products have a more narrow landing pad area between the legs and work great. 

I use Tranquility ATN (AllThruNight) brand for daytime instead of a normal daytime brief because they allow movement OR sitting/laying down under high fluid capacity without leaks.

At night I use different diapers based on fluid intakes, amount of each, and flow of either form of incontinence before bed. Some nights I can get away with a simple Depends Tabbed Brief with a booster, others require more protection such as Seni Super Quatro Lv 9 protection briefs or something as strong as NorthShoreCare's MegaMax brief holding 6,000mL

awesome