I think planting little seeds over time and being there for her when she decides it is not the truth is the best way to go given the delicate nature of the situation. (I don't want my parents deciding that I am too much of a threat to her spirituality.)
You are very wise to take this "avenue" in regards to how you treat your sister. It IS a delicate situation, and the last thing you want is to alienate her. She'll need you to be there for her.....when she realizes some day that the WTS is a false organization.
I'm doing the exact same thing with my family member. Planting seeds of doubt little by little.....and I believe the seeds will multiply & blossom into the realization that the WTS is a mind-control cult masquerading as Christian.
All the best to you,
Lavendar