Thank you for sharing your story and I am really sorry that you have had such a hard time!
I spoke to my Sister (who is a Genetic Scientist) today about contributing external factors or causes and she beleives that external factors are not to blame for the onset of this disease! So you can be certain that lifestyle wasn't to blame here (petrol or any other substance).
How she explained it was that, dividing cells can sometimes mutate as we age and cause diseases, so I guess for our loved ones it was just bad luck!