it’s not guaranteed that smoking hurts anyone. if you’re meant to die, you’ll die anyway, right? so what does smoking have to do with anything?
It is guaranteeed that smoking damages your lungs, however the age at which the smoker will feel its effects, and what effects they will experience is variable. The example you cited of your aunt and uncle is a random situation. Because one smoker lived longer than their health-conscious partner, means nothing in the big picture. There have been numerous large-scale controlled studies comparing the health effects of smokers and non-smokers and the results are clearly obvious that smoking is harmful!
With regard to the “you will die when you are meant to”, I am no rabbi but I do think it plays into responsibility. Do you want to be responsible as the cause of your own death, or would you prefer The One Above arrange it as He sees fit. At the same time, perhaps fulfilling the mitzvah of “Unishmartem meod linafshoseichem” will give the merits for a longer and healthier life than pre-ordained.