Well, this is the way I told my devout JW mother. We weren't in the same congregation, so for a couple of months, I didn't think I could tell her at all. Then it just couldn't wait any longer since she talks about the Kingdom Hall and Watchtower all the time. I started out by saying that I had never told her this but I had been miserable in "The Truth" for about 20 years. She seemed surprised. That amazed me since I had been inactive for years and barely went to the meetings. Anyway, I told her that I really tried very hard and prayed to do better, but it wasn't happening. She seemed okay when I told her that I was still going to read the Bible and live as a good Christian. I further told her that I would not try to make her leave, and I knew that she was very happy as a Witness. Then she started asking the typical questions about Armegeddon and the New Order so I had to tell her I hadn't agreed with what the Witnesses teach for many years. She said that she did and that they were Jehovah's people. I told her again that I was glad she was happy and but I wasn't.
She kept up with the objections and I decided I had to tell her that I had done some research and found out many disturbing and shocking things about what was going on with the Governing Body during our 38 years of being Witnesses. She got a little defensive, the typical, "you can't believe what is printed", "everyone's against Jehovah's organization", etc. I told her I probably know more about what is happening in Brooklyn than the congregation members, and I asked her if she knew who the current President of the WBTS was. She didn't, of course, but she thinks that's okay.
A couple weeks later, she told me she was going to talk to a brother that she could trust about me because it was bothering her. I told her about the risk of getting me disfellowshipped and that would effect my son more than it would her and I. I told her that since I had faded, a disfellowshipping would be like quitting a job and the employer hunting you down to fire you. She said she would talk to me anyway if I were disfellowshipped, she just couldn't talk to me about religion. (Actually, that would make my day )
On her way to see the brother, she rear-ended the car in front of her. She was fine but the car was totalled. I was glad that she wasn't hurt. I love my mom, and she loves my sister and I very much. It was pretty ironic to smash up your car and walk away from it on the way to have a "spiritual" conversation with an Elder. I almost suggested to her that it was a sign that maybe it wasn't a good idea. I don't think she ever followed up with that brother, she hasn't mentioned it since.
You know your family. I would say to mentally prepare for the worst and how to respond. It might go better than you expected. I expected my Mom to cry and carry on like I was dying. But she's fine. My thoughts and prayers are with you.