It's extremely difficult to get someone to wake up. It's a cult, your sister has been systematically programmed over a period of years to think a certain way and to be afraid of anything critical of the organization. I recommend Steve Hassan's Combatting Cult Mind Control.
Steve has helped many people who were in the Moonies cult (he is a former Moonie). Basically he tries to connect with the non cult aspect of people, then ask them questions designed to get them to think. Confrontation and telling people bad things about the Watchtower never (ever, ever, ever) works. Direct confrontation just causes people to become fearful that they are being misled by Satan.