My sister's 13 year old cat has started drinking a lot of water and so peeing a lot, she appears to be quite well otherwise but wonder if there might be something wrong?

That is normally a sign of illness, I would take to the Vet to have it checked out.

You can read more about it here:

http://pets.thenest.com/symptoms-cat...ting-7234.html

Get her checked out. Might be kidney problems. This was the first thing I spotted with my then 17 year old.

Was the vet able to treat your cat successfully?

Sadly he got progressively worse and we lost him about 7 months later. He was put on a specific diet and medication and a couple of times had to go in for blood tests and injections. A lot of older cats do get kidney disease so please do get yours checked out.

Thanks.

I have already told my sister that her cat needs to see the vet, I am trying to be encouraging, but sadly kidney problems are probably a red flag for an unfortunate outcome.

daisydee - if that is the diagnosis (may not be and no point jumping the gun) then there is a very good website with a lot of information to help make life easier and make informed choices http://www.felinecrf.org/
What I noticed was that my cat who had always drunk a bit suddenly started slurping away at water day and night, presumably as his kidneys were no longer able to do their job. Hope things go OK for you and your sister and kitty.