I'm scared I'm going to out myself as ignorant because I've not any real world experience of dealing with this from a relative.
I do however wear incontinence underwear, at 27, due to birth injuries and a very slow, very uncaring system.
Could she have hidden worries about her own eyesight that mean she doesn't feel confident using liners that she's not opening up about. They're a bit fiddly sometimes and it's honestly why I opted for these rather unsexy disposable incontinence underpants.
I don't blame you for how you feel at all OP. I would be frustrated too, but it doesn't mean you don't love her, it's just not something any one ever really anticipates having to endure. And poo of all things around the house is not nice at all, even though it's accidental.
Second the options for a lidded bucket, treat her underpants like cloth nappies, get some napisan and Milton and soak them for her and do a load every so often. In fact I'd get a couple, one for her room and one for the bathroom(s).
IKEA also do some great, cheap, lightweight bathrobes you could get half a dozen of, and always make sure that there's a clean one hung on the back of the bathroom door so she's not got excuses of forgetting.
The idea of cutting a puppy pad to the shame of a toilet floor mat was an excellent idea from pp too, just for extra security.
If, after all this work, she still doesn't cooperate, considering she's of sound mind, I would have a discussion with the next steps and what you're willing to and not willing to tolerate and the consequences of it as you've made reasonable adjustments to accommodate her in your own home and she's actively choosing not to use them.