You can tell the clothes are the problem when the zipper pulls at the same spot every morning, the waistband rolls down by lunch, or the sleeve stops short at the wrist. Nobody argues about a shirt like that. It just gets worn grudgingly, or it sits in the drawer, and somebody ends up buying a replacement that is also wrong.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Measure the body first
Start with a tape measure, not the tag. Take the measurements her clothes are actually built around: chest or bust at the fullest point, waist at the natural waist, hip at the widest point, and inseam or outseam if pants ride up or drag at the hem. If she spends part of the day in a chair, measure the seated length too, since a pant that fits standing can bunch behind the knee when she sits.
Write the numbers on one index card and keep it with the household papers or in the closet. Sizes differ from one maker to the next, so an old label is a weak guide. The card is the only size you can trust.
Check the cut before you buy
Look for the features that decide whether a garment is usable, not just whether the number matches. Does the opening go wide enough to get over her head or arms without a struggle? Does a front closure, a wider neck, or a stretch waist make dressing easier for her? Is the sleeve cut to let the arm move, and does the hem land where she wants it? Read the size chart on the maker's own page and compare it to your card, because a medium from one maker can run as a large from another.
Choose fabric you can wash on the schedule the household actually keeps. A piece that needs hand washing will not get washed, and then it will not get worn.
Buy in pairs, not in piles
Rather than refilling the whole closet at once, pick one garment type to test first: a pullover, a pair of pants, a robe. Have her wear it for a week. Notice whether it goes on and off without help and whether it still fits after a wash. Then buy the rest of that type. It is the easiest way to avoid paying twice for the same mistake.
Decide who buys, returns, and keeps the receipts
This is where the money quietly leaks. One person orders online, another picks up the parcel, a third keeps the receipt, and by the time the wrong size arrives the return window has closed. Settle the roles before the first order. Name the person who buys, the person who handles returns, and the person who holds the receipt and the tag. Write the return deadline on the index card next to the measurements.
Keep a simple running list of what was bought, what was returned, and who paid. If you are the one fronting the cost, write it down the day you spend it. That list is what lets the family split the bill later without a fight over who owes what. Notice, too, whose hands are doing the sorting, the returning, and the driving. Put those tasks on the list as work, not as favors.
Know when to stop adjusting and start replacing
Clothes sized for a different body usually cannot be altered into a good fit. A tailor can shorten a hem or take in a seam. A tailor cannot move a neck opening or swap a closure that is hard for her to reach. If a garment fights her every time she puts it on, the answer is a replacement in the right cut, not more adjustment. Set the outgrown pieces aside for donation so they stop crowding the clothes she actually wears.
Where St. Louis help fits in
If your mother uses a wheelchair or has trouble standing for long, a trip to the store can turn a simple purchase into an all-afternoon job. Look for a shop with a fitting room large enough for a chair, or an alteration service you can drop a garment off at without going back in. The local help page lists resources in the St. Louis area that can point you toward options, and it is a good place to start if you are not sure who to call first.