It can become all consuming, Helen, same as somebody who is constantly on a diet. And totally boring and frustrating for those who have to listen to it.
Have you thought about telling her that if she is not prepared to take medical advice and stop eating the wrong food and control her weight you will not discuss diabetes with her again? You don't want to hear another word about it. Harsh, but perhaps it is necessary.
Otherwise let her ramble on and moan but take no notice. Don't comment or react.