Thanks for all your comments.
It is possible that she was dehydrated, as she spent so much time asleep. However, as I explained in my OP, everyone who visited would make a cup of tea and drink it with her, and she was even drinking water readily - sometimes preferring a glass of water to a cup of tea.
There is no point in asking her about any memories. She wrote her "Memoir" a while back. But now, it's all gone, or rather she hasn't got the strength to recall things. For example, I asked her if she smoked whilst she was pregnant with me and my sister, and she couldn't remember. (She probably did - she smoked from age 17 to about 70)
Since my first post, she has got worse, and has been in hospital for the last ten days on antibiotics IV. Only the day before yesterday did she start talking normally again.
She is considerably weaker, and they have also found a chest infection (which she sort of already had - not contracted whilst in hospital) and a heart arrhythmia, which she has also had in the past.
My sister says she can't manage more than a couple of words without getting out of breath.
The hospital staff are being wonderful and she is being assessed for a care package, but DS is looking for a suitable home to put her in when she is discharged, as we can't leave her alone any more.
I miss my nightly phone calls to her, and find myself reaching for the phone at around 6 pm most days.
I wish I could do more from so far away, but I am flying over next week and shall be there for 9 days. No knowing yet if she'll be at home whilst I'm there. Most likely not, so I will be spending the time at her house, bagging stuff up, as it is unlikely that she will be returning there.
It hasn't really sunk in yet.