@sa4984, you're right. @OldMarriedMan, if you traded places, would you want to wear a diaper?
Consider calling it what it really is: disposable underwear.
The ones these days in the drugstore are designed like underwear. They have colors like underwear. Yeah, you'll never see them in a lingerie catalogue, but they aren't built like babies' diapers or toddlers' pullups either.
Your stepmom already is embarrassed and feeling like a little kid. The bedside commode definitely will help when access is of the essence.
Sometimes a bit of cool breeze is enough to tell the bladder that it's time, right now, especially if she has to be on some sort of "water pill" such as hydrochlorothiazide for high blood pressure. (Do check with her doctor to make certain about side effects of any of her meds. Sometimes substitutions with fewer side effects are available.)
If she's willing, let her pick out the colors and design she wants on the underwear while still making certain that it has the absorbency she needs.
If the bedside commode works without the disposable underwear, even better. If she likes wearing dresses, that, too, will help with getting underwear out of the way faster when getting on the commode or a regular toilet.
But having both the commode and disposable underwear around could allow her to get up and about with more confidence than she's comfortable doing now. Sometimes even exerting the muscles to stand up and get walking cause other important muscles to loosen up.
I hope she sees the advantages. My mom actually went a different direction: Instead, she liked the idea of a pad in her regular underwear because she considered it more like the menstrual pad she hadn't needed for a couple of decades. Most of the time, that got the job done, but it wasn't perfect.
So think about changing the lingo. Give your stepmom a choice, and take care.