I too was never a JW and never studied so you are not alone here. It was a JW friend who brought me to this site. Here is a web site that may help you out www.freedomofmind.com
Freedom Of Mind was founded by Steven Hassan, an ex-Moonie who is now a licensed Mental Health Counselor. His specialty is exit-counseling. His books, Releasing The Bonds was written for people like yourself who have a loved one stuck in a mind controling group.
Rather than tell your son and daughter in law the facts of the Watchtower (changed teachings, failed prophecies, etc), put these facts in the form of logical questions. Logical questions will make them "think" of an answer. Your daughter in law is already indoctrinated but you may be able to reach your son with these questions.
I have never been a dub 'neither. I have a friend who is, and like you, the need to understand her thinking (beyond just what could be understood from their literature) landed me here. As said, things are a bit more intricate than it seems on the surface. I advise a lot of listening in order to keep the communication open. But when a door for a real opportunity presents itself, walk thru it, just like you did with your son...
I hope to see you around here often, especially with the updates...
So nice that you have been lurking a while and decided to come in and visit. I hope you stay a long time and keep us updated on the situation with your daughter and son-in-law. With so much advise from others I to am just thinking that by coming in here you will get all the support you need. We have wonderful friends here and loving and caring towards all.