Maybe let her know that you will share the weed,as well as teach her how to use the 'box. A little honesty goes along way. Could be a bonding moment.
If that won't fly there's always water-boarding.
You might have been able to come up with something if you didnt leave the weed in it. You may be out of luck unless your parents are ridiculousy ignorant/gullible.
If she took it with weed in, then honety is probably the only possible way to go. But I see you say "i think", so you are not sure if she actually took it. Could you just misplace it or something?
tell her that if the battery is in she has to keep inhaling otherwise she's gonna combust and with the mflb combustion is really not a pleasant experience.