It is easy and, I guess, natural when one gets lung cancer to want to point the finger at something or someone. Lucie, my wife, was raised in a house without any smoking, and we never had smoking in our house. She got lung cancer.
I on the other hand was raised in a house with a lot of smoking, though I never smoked myself. I didn't get lung cancer, nor did any of my parents or siblings. Go figure.
I agree with others that focusing on the smoking does not rally our cause. It only gives people an excuse to ignore us. We are living with the uncertainty of why my wife got lung cancer, and we have stopped rehashing why it could have happened and have gotten on with keeping her well. Hope your sister can do the same. Don