science

It is a little more than cutting sugar. If you replace all your simple sugar with complex sugar you will have an improvement in blood sugar control, that is true. You won't reverse insulin resistance with that change because you will still have elevated insulin levels all the time.

What ketosis allows is a period of time where you are not using large amounts of insulin to push your blood sugar down to safe levels. Instead you use gluconeogenesis (making sugar in the liver from fat) to push blood sugar up. This means you don't overshoot and have to push it back down.

If you stop using insulin all the time your body becomes less resistant and it becomes more effective, allowing you to actually push your blood sugar down when you need to. This is one of the core issues with diabetes and can actually be solved.

As for your wife's mum, what she is eating is up to her. She is an adult and has to make her own decisions. If she chooses something you wouldn't choose it isn't an indictment of your and your choices. She can do what she wants and so can you. It isn't a personal insult. If she chooses to take keto very seriously and won't eat beans then treat it like you would vegetarianism, don't be dismissive or rude, just be a supportive person and don't make bean (or other high carb) dishes when she is around for dinner. Or make the high carb dish as well as something for her. Or just make parts of the meal keto and others high carb and mix and match on your plate. Or don't have her over for dinner. Lots of options that don't require you matching her diet or pushing back on what she eats.

Diet is personal. It is personal for her, and yours is for you. Her diet is not personal for you. You don't need to concern yourself with her choice of food just like you don't need to be concerned with her choice of underwear.