Your argument will have to appeal to their sense of wealth. Explain to them how lucrative a business selling marijuana can be. Explain to them how growing marijuana can strengthen the bonds of your family. Get them interested in the complications that can arise when taking on such a feat. I want the entire family to grow like they've never grown before!
Why not?
Rules.
Blasphemous rules!
It's illegal, but we will go down as a family who, through a tiny seed, built a castle that law cannot decimate with it's conforming touch. Open your family to the idea. Let them know and don't allow them to push you away by crushing your dream. A family sticks it through and together you all can construct the most foolproof method of growing the magical herb.
It would be a family thing. Now how fantastic would that be?