Your niece sounds like me lol -when I was her age I was the same way. I didn't go away to college and lived at home until I got married. I did have the occasional drink when I was 18 -25 but TBH I just didn't enjoy drinking plus several of my family members had alcohol problems - one brother was an alcoholic. Most of them also smoked and I grew up hating the smell. I know I'm boring lol. So I decided I didn't want to drink alcohol.

Regarding your niece - if she has a leader type of personality, she'll be fine and won't be influenced by others doing those vices. If she's impressionable and been unwillingly sheltered too much, then she may experiment. If she hasn't done any of these things so far, I suspect she's made the choice not to get involved in those things. For me it wasn't a moral thing as much as a health thing and I wanted to go the opposite of my family members. I feared becoming an alcoholic too. Have you asked her how she feels about the things you're worried about?