I agree with Wade. In my experience, the vast majority of parents out there, don't take the time to teach their children how to think, they just simply tell them what to think. As a trusted member of the family, you can have a positive influence on the kids, and thats a great thing. Having that influence is both a privelage, and a responsibility. Take the time to show them the truth about land use, and help them learn how to make decisions for themselves, based on what they want to be able to do (like riding ATV's and sleds), and now you're teaching young people how to think, and make decisions based on information, not emotion. But, as I am having to learn myself, a little bit at a time! I'm still working on that one.
And.... I'll bet that your sister gets some serious influence about her views from those goofy folks you met the other night. If you would like your sister to think more like you, try to spend more time with her, and you be the main influence (by example, kindness and grace) not her lazy hippie friends. Be genuine, (they're chicks, they can tell when we are genuine or not) and you'll likely get an oppertunity to share with her the facts about the land-use argument. You'll likely make your sibling relationship better, and possibly gain a voice for our causes.
Merry Christmas!