Way back in the 70's when I first became interested in nutrition and how different foods affect the body, I bought a book called 'Pure, White and Deadly' by Dr John Yudkin. It's still in print and a quick Google indicates you can pick up a copy for as little as three pounds.
You can be as slim as you like but if you load your body with refined sugar, you will not be living a healthy life and in your later years you will wish you had known this info long before.
It is nothing to do with weight.
I know the fashionable stand is to assume it is but believe me, I know differently.
The sugar industry increasingly promotes white sugar products to the detriment of children's health because they know if you get a baby hooked, it will probably be addicted for life - and sugar is addictive.
Using the specious BMI, you can be judged many kilos overweight, as are many 300lb+ NFL players, weight lifters and even the All Blacks but if you keep active, have a good diet which cuts out most processed foods, including sugar, you will be as healthy as you can make yourself.
en.wikipedia.org/wiki/Pure,_White_and_Deadly
I don't think you should tell your family outright what damage they are doing but if you buy the book and read it, you could then leave it about or give it as a Christmas gift - along with some good quality honey whilst they are trying to get off the processed stuff