??? your entire family smokes weed, yet they don't want you to? your sister knows every toker in the state, but you can't get weed? sounds like bad communication to me...I'd let your sister know that you need a steady hookup, if she's trustworthy enough to keep it from your folks...

...although I would simply tell them, 'look, I know you all smoke weed, I've already tried it myself, and I like it. I'd like to start growing, and it'd be much, much easier if I didn't have to hide it from you all. if you can help me out, I can share my harvest...'