Tinker, give her time. It's hard to change the habits of a lifetime. My MIL was diagnosed when she was in her sixties and it has taken her several years to improve her diet. At first, she didn't realise how important it was to change her diet and she found her dietitian's advice too strict. When dh was diagnosed, he joined Diabetes UK and we started passing on their magazine to her once we'd read it - it's quite informative and I think helped her to understand how she had to change. Her dietitian had given her the impression that she had to cut out all the things she particularly liked - in her case chocolate and puddings. But actually, that's not the case. For instance, chocolate and puddings are OK as long as they're eaten after a main meal (because of the way sugary food is digested - if eaten on it's own, sugar is quickly broken down into glucose and it passes through the stomach wall resulting in a sharp rise in blood sugar (and in untreated diabetics, blood sugar levels stay high because either no insulin is produced or it is ineffective). If sugary food is eaten after a meal, the stomach is already full and the digestion and absorption process is slowed down because the stomach is already `busy', so you don't get a sharp spike in the blood sugar levels). Of course, eating lots of puds and chocs isn't conducive to losing weight but I think it's important to make sure the diet isn't so restrictive that it seems impossible to stick to. It sounds like your mother has a weakness for crisps. She can still eat them but not too often. Maybe you could suggest changing to a lower fat variety.
I don't know your mother, but I would suggest you do some research, maybe join diabetes UK on her behalf so you receive their magazines, and get her well-informed. Then it's up to her. I mentioned a book called `Diabetes at Your Fingertips' earlier - that might be useful for you to read. The ISBN is 1-872362-79-6.
And yes, the blurred vision is related. It happens because the lens of the eye gets swollen when diabetes is out of control. This is reversible (it should improve if she gets her blood sugar levels down) and is different to the retinal damage that can be caused by high blood sugar which is irreversible. Your mother will need to have special eye tests to check for retinal damage. This involves the optician putting drops in her eyes to make her pupil dilate. She won't be able to drive afterwards as the effect of the drops takes a while to wear off (also, she may need sunglasses afterwards). My dh had the blurred vision and was advised to wait until this had improved before going for the other eye test.
By the way, don't worry too much about your mother not telling her doctor the whole truth. Every time she goes for blood tests, she will almost certainly have a test for glycosylated haemoglobin which will give the doctor and idea of how she's been controlling her diabetes over the last few months. So she can't just `be good' for a few days before her tests and get away with it!
By the way, did you develop gestational diabetes? If so, then you are definitely at risk of going on to develop diabetes. If not, then you can't be sure either way.
Sorry this has been such as essay - it's one of my hobby-horses!