Shades of grey are the first things that come to my mind when confronted with a situation like this. It is so difficult to determine what is right and wrong, as the issues don't fall into a black or white/ right and wrong area very easily.
On one hand, your parents are increasing in age, though in good health, and we have no idea how long they will be around. Tell them, and it may be too much for them to handle, especially after spending so much of their lives involved in the organization. They may feel that they are in no position to even consider the fact that this is not "the truth." But, not telling them could be perceived as deceiving them, which I imagine does not rest well either.
The rest of the extended family might be easier to deal with, as the relationship is not as close as it is with a child and parent.
I am in favor of the slower fade for several reasons. We just don't know what the future holds. Will there be changes within the organization to allow more freedom in relationships with "unbelievers" or perhaps someone on their own will come to the same conclusions as we all have. One of the challenges we faced is the fact that no one is allowed to talk to another about doubts, so no matter how close someone is to us, we may never know their real thoughts. Talk about a frustrating situation.
However, with the slower fade, the WTS will allow them communication with you, so then you will have the opportunity to slowly drop thougts or comments with them, during the times you do see them or talk, which will be fewer and fewer with the distance. I like the aspect of time solves a lot of issues.
At the same time, there is the moral part of it, where we ask ourselves, do we have a moral obligation to let them know what we found out? I have felt that way with my son. He says that he wants to know why we don't go, but I am not convinced that he can handle what he will find out. But perhaps the way to let them know, is by doing the slow fade, as that will allow more opportunities to discuss it at a later time.
If you did talk with the relatives, it seems as though it would need to be done each one by themselves. If everyone finds out at the same time, it is too easy for them all to gang up and not listen to you.
My wife and I are putting together a large binder full of all the ammo we need to defend our beliefs that this is not God's organization. We will have it ready for the time my son comes to visit, and will be quite helpful for any witness who wants to confront me on a ton of issues, friends or not.
I simply cannot recall all information I have found over the years off the top of my head. So to have this binder together, easy to read, logical, with proof from their own books and magazines, should be very powerful.
If you do confront the relatives, make sure you are armed with all the ammo. You already know the bible very well. Take that knowledge along with what you have found out, I have no doubt you would cause some concerns in the minds of the relatives that you talk to. But it will be easier to do when you have simply faded, or become "spiritually weak." I personally have allowed that one to be said about me for years, and it works quite well so far.
I feel your pain and stress over the situation. Regardless of the outcome, I am confindent that you will make the best decision that you can.
Best wishes.