Your sister has mild dementia. One of the signs of dementia, even mild dementia is a lack of awareness of their surroundings and often a lock of awareness of themselves.
Your sister's behaviour is entirely consistent with the condition she has. You are fortunate that it has not yet spread to personal hygiene and dressing, and eating, or rather not eating. Sadly these are to come.
Whether her dementia is caused by Alzheimers or not. It would probably help you to get in contact with the Alzheimer's Society www.alzheimers.org.uk/ . They offer support and help to the families and carers of people with dementia and also help and advice about how to cope with any problems. I think you would find them very helpful.
In the meanwhile, I would ignore suggestions from the men in the family. They mean well, but it would be a disaster.
Perhaps when you visit you could do a bit of discrete cleaning, If you go to the loo, stay an extra 5 minutes, and do a bit of cleaning or wiping down, perhaps you could go into the kitchen when she makes a cup of tea and casually wipe some kitchen surfaces down or enlist her help to get the washing up done.
But most of all get incontact with the Alzheimers Society, they have so much information about dementia and its progress, which, sadly, is slowly and inevitably going to get worse and the more you know and understand the better it will be.