Thank you Violet Jane for that very comprehensive and incredibly useful reply - just reading it confirmed all my fears and together with what other people have written here has made my mind up, which was what I needed.
She is currently on a dementia ward while her husband, my stepdad, undergoes a further op to have as second stent fitted, the hospital see her as a bed blocker, which of course she is, and are looking to either persuade us to put in care so she can be discharged when her husband is, probably next week, or find a respite place in a care home. I now am gong to fully push for the latter.
The crisis that is happening to them is what we feared would happen and I think we must use it as an opportunity to move her into a care home.
Such sadness. Thank you all very much.