Point out contradictions in their legalistic rules and how they can be biblically contradictiary.
For example, the whole "We're still married in the eyes of the lord." doctrine. That's a good start. Then there's the question of "Would those that disobeyed the society about (Insert doctrinal gaffe here.) be disobeying 'Jehovah', even though according to the WT, the disobedient one was always right?
Then show how contradictionary the legalism is with Christs' message of a less legalistic way to god, and you have them thinking. When they start thinking, then lay on the heavy hitters, like the UN and child molestation. Why? Because witnesses sometimes go through these stages when they're getting out.
1. The society is wrong, but they're the closest thing to the truth!
2. A godlike organization shouldn't be so legalistic, but I'll stick with them because I have family in.
At this stage, if the family finds out about the doubts, they go through these stages
3. Maybe they don't have the truth, but they have good things about them.
4. Ok, they're legalistic, but they're still following god.
5. They're not following god, because he wouldn't like that to go on in his congregation.
You lay the heavy hitters around stage 5.