Hi Michelle,
I understand your dilemma completely, since being diagnosed myself , every time I see one of my friends with a lot of weight round his middle but telling me everything is " fine" I want to try so hard to stop them getting into the mess I got into. Some people listen, others don't .
It runs in my family too, all of us are weigh too much despite different lifestyles.
This can be very very hard to do if someone won't listen to you.
As a youngster, you probably have a very limited budget, but if you can find a way to afford it, could you rope them into doing a study for you in your diagnostics ?
there are two ways of doing this :
If this was in the UK / France/ Australia I would buy a libre free style plus just two sensors. I'm not sure if these are available without prescription in the US? though as diabetics it should be reasonably easy to get a prescription if necessary ?
in the Uk you can buy the product and also then download an app that lets someone else monitor the results remotely as long as the user actually uses the scanner every few hours.
If not then you can do the same experiment, cheaper, though it requires more complince from your in -laws - by testing blood sugar with a finger prick test on waking, just before starting each meal and 1 and 2 hours after each meal.
Older people can be more inclined to listen to youngsters if it is presented the other way round.
for example you you could ask them to help you in an experiment to look at the impact of certain foods on the diet of diabetics and you want to measure both of them to do that as they are both diabetic ,particularly as one is a slim diabetic and the other presumably more overweight?
If they would be willing to participate then you could ask them to wear the libre once each for two weeks. - if you can get hold of one - I know they are easy to buy privately in the UK -
https://www.freestylelibre.co.uk/libre . the sensors cost £59 each in the uk and I think you can simply down load an app to then measure it using a phone .
The experiments would be - have them eat as they normally do for two days. Then ask them to eat very little for a couple of days , then ask them to eat from an LCHF diet list for a week, e.g. using a keto food pyramid which you can get numerous examples off the internet including or excluding dairy etc depending on the types of foods they do eat.
Then eat the "normal foods" for a day and then go back to LCHF for the remaining 3 days.
If your in -laws are anything like most of us - the first two days would show very high blood sugars and big spikes, the two days very low calorie eating should bring them down a fair bit, the weak of LCHF should show them stabilising, the one day normal diet will create bigger spikes again and the final three days on LCHF will show them stabilising again. .
Make sure that the LCHF food is actually fatty - ie bacon and eggs cooked in butter , good mix of olive oil, butter cream, full fat cheeses, meat , oily fish , buttered above ground veg, salads with avocado and nuts and olive - berries and cream, whatever you think they are most likely to like but no flour, rice, sweets, pasta, or anything sugared including high fructose corn syrup
In order to get the data for two weeks, all they need to do is remember to actually scan the disk every few hours, and to write down what they are eating. Would it be possible to arrange it so that you actually cooked for them for a month ? which would help measure compliance.
Either way you will be able to look at the data - which would be fascinating for your studies anyway,
( and indeed its something I would love to do myself with a person switching to an LCHF diet because I think that changing diet has much faster results than most people recognise based on my personal experience and what I know now even though I did not have a monitor at the time).
It would also show them in stark relief exactly what is happening when they eat the wrong foods and so give you some ammunition to help them manage the condition for themselves.
hope this gives you some ideas
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when I showed her everything I had written down she started crying. I walked in last night to her eating a salad which was a definite first for me. I hope she continues 
