well I'm in the same boat as you. My sister just turned 18 and I'm nearly 28 myself. She isn't really into it though to be honest but I would definitely feel wierd about it, but now that she's over 18 it makes a little difference. I have a two year old and I would MUCH rather her come to me when she is in her teenage years and tell me that she is smoking rather than drinking. I've never really seen a violent pot smoker who goes and gets in their car and drives around (me I"m a lazy smoker lol) and ive never heard of pot poisoning like alcohol poisoning. But I don't think I would ever smoke with my daughter not until I was like ninety and takin it medicinally and she brought me some at like her age of sixty something haha
But I don't think I would punish her as a teenager for smoking, but I wouldn't let her do it , say , in her bedroom either.