The brother in law analogy is pretty emotional and sentimental. So, you say that if you don't speak the FULL truth to your brother, then somehow he will get the truth from an intrinsically chaotic "mass"? How will that work? Chances of this happening are slim.
Meanwhile, if you speak the truth, he may well convert fully. Or he may not. Chances of his converting are still slim, but they are greater than above. If you aren't going to talk about the TLM at the moment, when ARE you going to talk about it? What is this PERFECT moment that you are waiting for? No such thing as a perfect moment; you do it when you can. What if he dies tomorrow?
It's none of our jobs to "project" whether or not someone will convert. Our job is to speak the truth, not give it piecemeal to whom we see fit. Who would have bet that St Paul would convert when he did?
I understand that there are times when it is pointless to talk about the Faith to people, but these are usually times of extreme suffering or emotional turmoil. These can be a barrier (in the short term) to discussions, because the person is preoccupied with stress. But outside of this, when people are just living out their day to day lives, THAT's when they need to hear the truth. It may not sink in right away, but it will eventually.