Poorcraft
As a 21-year-old, I've gotta say this is a risky one. I would hate to be handed Poorcraft because somebody thought "oh, you're 18, you'd love this" - I'd be bitter that I wasn't just given a real book like any other human being. On the other hand, from what I've seen, the subject matter is really pretty relevant and tailored to folks our age. So it would really depend on the individual - I wouldn't get it for a cousin I barely knew, for example, but OP knows his/her sister well enough.
Your Money Or Your Life.
This one, for real. Or the MMM blog, though that's not really a gift. What's the point of saving money if you don't also have a value system in place that makes you realize you're literally getting years of your life back? Why would I cut cable and cook my own dinner if I thought I would be working until 75 either way?