If you haven't already, you might point out the Biblical test for a false prophet, found at Deut. 18:22.
If you have, and your brother-in-law agrees that the Society has falsely prophesied in the past, you might want to try this using the following from the Reasoning book, page 323:
Because the Society teaches that the soul is not immortal, which make it the true religion in your eyes, is that reason enough "to drink a glass of poisoned water because someone assured you that most of what you were getting was water?" Or is the poison of false prophecy enough to make you want to assure yourself that there isn't any more poison in the glass of water offered? Then kindly explain that this is your dilemma.
Of course, this approach will not work if you have already railed against the Society and bombarded your b-in-law with all sorts of information already.
Another thing that may help is Randy Watters' "Opening the Closed Mind" which you can find at
the http://www.freeminds.org site. As his opening paragraph states:
It is rare to find a Witness who has not seen or heard information exposing the dishonesty of the Watchtower. Why, then, do they not see a problem? Evidently, something else has prevented them from objectively analyzing factual information. Their minds are trained to stop short of doubting the organization - a wall has been erected which says, in effect, "This far you may go, and no further."
This is where your brother-in-law is at, and it's frustrating. Randy's article will suggest how you may sensitvely try to extract your loved ones from the Borg. I think you'll it find very useful.
Be Gentle. Patient. (Hard to do, I know. )