What you should do:

First, you print off a bunch of bs facts from the gov and sites like above the influence. Then you call your sister into the dining room with the pipe and cannabis on the table, and with you most serious face, say "We need to talk", and hand her the bs fact sheets. Then your going to start lecturing her on why its wrong (You can judge how far you want to take this part). Maybe you could cry a little and be like "I looked up to you...sigh". Then you laugh, say just kidding, and go blaze with her.