Thanks all - very helpful. 
WhoseGot - that sounds depressing, sorry to hear about it. 
madmomma - yes, I will eventually just pass it on over to my brother to deal with. You're right it's his problem - but I do see where he's coming from, it's his precious first born and he's worried, and now I'm worrying about their health too. Difficult.
math - thanks for taking the trouble to give such a detailed reply, you're very kind! To reply: my mum is I think depressed, but her mental health is a whole other big area. It's certainly not new, and we've never managed to get her to see a doctor. They don't really have friends round (once in a blue moon), but they have spent a lot of time and money extending the house and updating the inside so that family can visit - I think they are disappointed this hasn't already happened more, which is partly why I want to work out how best to tackle these issues, which may be part of the problem. To be honest, DH and I would want to visit more if we weren't expected to share a mouse-riddled single bed - so I can only imagine what other less close family members feel like.
Cleaner - mum would never. Until a couple of weeks ago she worked about 30-35 hours a week but sees very much as part time and doesn't understand why the housekeeping is hard work (she cooks everything from scratch, supermarkets are half an hour and more away - to name a couple of reasons!). She'd feel she had utterly failed if she had a cleaner.
As to babysitting - she couldn't kneel when she had us, either. She managed. It is worse now, but I doubt she would find it too hard, especially with my dad around. She is very, very keen to get involved.
I feel awful because reading these posts (I am not criticizing at all, just noticing this), people are responding as if my parents are old. I guess they are - but I wasn't quite ready for it.