I have failed miserably at this myself so I cannot offer any advice on what to do. Maybe more what NOT to do.
I tried to reason with her on the doctrinal stuff such as baptism to the organization through their two baptismal questions, Jesus not being the direct mediator for the earthly sheep (only the 144,000). How the WT got it wrong on the 607 BCE date, the pyramids and how Pastor Russell was originally preaching 1874 as the year of Christ's invisible return. I told her about the Candice Conti case involving sexual abuse of children and how they covered it up. I've even recently pointed out the WT UN scandal when a CO was giving a talk linking the wild beast with the UN. She does what she wants, ignoring WT rules when it suits her, following WT rules when it suits her.
She only clings to the religion because she has family in it and believes it was instrumental in helping her live a clean life during her formative years. She does not realize that the same thing has been said by others of other religions but stubbornly clings to the idea that churches allow unmarried people to live together or are all about tithing. She has a VERY superficial WT view of other Christians that can easily be demolished if she ever listened to the stories of former devout/fundamentalist individuals and how they actually lived. Not to mention the fact that the WT has its own sins (as I stated in the prior paragraph) which effectively negates them from being any more moral than any other Christian church.
Bottom line, facts are powerless to the faithful.