I think that the best way you can prove to your family that marijuana isn't as bad as the government lets on is to just be as responsible as possible. Prove to them that weed won't fuck up your life by getting as good of grades as possible, maintaining a decent job, etc. Just don't fuck up and give them a reason to say "I told you so" and eventually they'll realize that they're out of line and miseducated. I have the same problem with my sister, and I know that the only way I can show her that weed isn't that bad is to remain a good person and show her that "stoners" aren't bad people.