My mother was much younger, 70s, but had heart failure resulting from her diabetes.
Sometimes she would get fluid buildup in her legs and, crucially, lungs and heart, leading to breathing difficulties, and then would have to be treated in hospital with diuretics. This would have an impact on her kidney function.
After treatment she would be ok for a while and then it would happen all over again.
The last time she was in hospital for several weeks; released but on oxygen; deteriorated after a weekend at home; was readmitted.
This time her kidneys tipped over into official failure.
What did this result in? Well, keeping the heart and lungs going is always prioritised at the expense of kidneys, because you need them at all times to stay alive.
But if the kidneys aren't working, not only do you get a buildup of fluid (in the legs at first), but, crucially, the electrolyte balance in the blood goes out of whack. If the magnesium and potassium levels are wrong then the heartbeat is affected - arythmia - and that can lead to sudden death. This is what happened in Mum's case - the palliative team were starting preparation to get her home, but she fell unconscious suddenly while the nurses were providing personal care. They rushed to get us back in her room and she died in our arms minutes later.
So my guess is either your great Aunt will just pass away fairly quickly one day because she's 91 and the chance of stroke or heart attack or just "who knows" is quite high, or, one of the times she goes into hospital, they won't be able to make her better and she will pass away then.
If you can get her to set up health PoA it's helpful. I found as a general "talk to me!" document to wave at folk in hospital it was useful. But more concretely, if she has delerium from a UTI, if she's been incapacitated by a stroke, you can then represent her on health matters. At the very least, when you're a frail old lady stuck in hospital, probably catheterized and at the mercy of the staff, just being able to see a healthy person with a notebook and pen in hand fighting your corner is a great morale booster.
"Just talk us through what that's going to mean"
"How is this test result going to change her treatment or chances?"
Also if she wants she can do an Advance Directive to give you and the medics clarity as to what she wants in situations where she can't speak for herself.
On a personal note: after Mum's legs started swelling, I realised the clock was ticking. I arranged with work to take a day of unpaid leave a month (on average) so we could have days out, meetups etc (we didn't live close to her). Definitely worth it.