rowanwitch, its virtually impossible for one person to care for a person with dementia for any length of time. I know this professionally and also personally. Live in carers have to be employed in twos or threes so that they can get a break and residential home staff similarly. Its also amazing how badly affected someone can be and still be able to wash, dress and feed themselves. I knew one lady who even went out to a club and to her hairdressers. The secret was that her whole life was in a tight routine. She travelled everywhere by taxi who knew when to pick her up and where to take her. her shopping was done and delivered by her daughter and she only ever watched one TV channel. She got up when the alarm went off and went to bed when she had seen the news. To strangers she appeared completely functional. Sadly it all went to bits when she got a chest infection.
I do feel a bit for your sister. I have a dog who I won't/can't leave and have never put any of my dogs into quarantine since we had to do it on return from the US. When my mother was alive and I travelled to London to see her, I left the dogs with my husband, and he did the same when he went to see his mum and stepdad. I accept that its hard to understand for people who make different choices.