Hiya - it's your genes! Yes being overweight can certainly 'encourage' your body to object but it isn't always the case even if people are 'morbidly obese'. It's more than possible to be TOFI - ie Thin Outside, Fat Inside - this means that your own body is working against you by laying down fat around your internal organs instead of or as well as, underneath the skin and thus causing those inner bits to become sluggish instead of mega fit raring to go and working perfectly.
Best thing you can do to help yourself and therefore as a side effect, your wife - is to take this as a wake-up call (a kick up the bum!) by starting to pay more attention to your own body's needs. It's not a difficult concept (although the inner workings of the human body certainly are) because the simple explanation is that the body converts everything we eat or drink, into glucose - cos every cell in our body, whether it's in the right big toe, the heart, the skin or the brain, uses glucose for fuel to make them and therefore the organs they are in, function.
It's easy for the body to extract glucose from carbohydrate therefore if there's plenty of it going in, it'll always convert that first in favour of converting the protein or the fat we consume. Hence to begin with, it's the easiest to start by trying to reduce the amount of carb in the diet. You won't be able to reduce it to Nil and no-one's suggesting you try - but certainly, cut it down. In the days when a meal was meat spuds and another veg, you cut the amount of spuds down. If you use thick sliced bread, change it to thin or medium sliced. Change having a 'Large' loaf to a 'Small' loaf. Absolutely reduce or remove sugar in drinks, cos sugar is almost 100% carbohydrate whether it's stirred into a cup of coffee or in full sugar soft drinks - almost all the popular soft drinks have 'diet' versions.
If you have rice or pasta, reduce your portion and have a bit more whatever you serve with it, to compensate - eg Spag Bol - less spag and more bol. There are low carb/carb free substitutes for the rice and the pasta should you need to reduce the carb even more.
Anyway - enrol in the Diabetes UK Learning Zone and work your way through the modules at your own pace, to learn more about your diabetes. And jolly good luck with it, cos you are important!