Your sister should not have been snooping around, although it already happened so there is not much point in discussing it.

Generally everyone agrees that smoking is bad for your health, but it is still a choice that you get to make. I used to smoke when I was younger, hell everyone did, but I stopped when I was around 21-22 and have managed to live a healthy life since.

Smoking doesn't permanently affect you unless you smoke for years and year on end. There is nothing to feel bad about, when your father calms down have a relaxed conversation with him. Explain to him that you don't smoke much and explain your reasons. Then in a calm way, explain that you don't smoke in the house, or on the property, and that you just sometimes have a smoke with friends. Just make sure voices don't get raised because it will lead to a fight and that will accomplish nothing.

Hope everything works out, Luca.