The only things I can think of at the moment are to let your mum wear loose fitting, elasticated items of clothing - perhaps even in a bigger size than she actually needs, to avoid anything feeling restrictive.
Would she wear a kaftan type of housecoat when at home, to make things more comfortable for her, rather than trousers/skirts and tops?
Avoid blouses with collars and cuffs, get her soft tops with scoop necklines and either short or three-quarter length loose sleeves.
If her bra bothers her, get her those soft bras that can be slept in, rather than one that gives lots of support with wires etc, or if she only has a small bust, do away with bras altogether and use soft vests instead?
Also ensure that you use a very gentle washing powder and conditioner, so that her clothes are as soft as possible.
Wherever possible remove labels from the necks of tops - these can often irritate.
I'm pretty sure you will have tried most, if not all, of these possible solutions, but I can't really think of anything else at the moment. I'm sure other people will be along shortly with more ideas for you.
