Thank you for your candor and your kindness. All I have been reading seems to confirm your personal experience, that with an adult spouse "proxy" there is probably nothing to be done that would be of help.
I guess what I am trying to weigh is whether it is worthwhile to attempt any kind of intervention, even knowing full well that it is 1) unlikely to actually help and 2) possible that it will actually make things worse, both for my brother and for the family (I expect that if she even knew I were posting here and researching this, that would be the complete end of my ever being able to communicate with my brother again in any way).
What I am weighing against that is the possibility (which, the more I learn about this, the more of a real and present danger it seems) that my brother may actually be in mortal danger. It seems that, IF that is in fact the case (which is very hard to determine, since we have almost no contact with them now), that we have a moral duty to do SOMETHING to try to prevent what may in fact be real injury, or even eventually murder, perpetrated upon my brother.
On the other hand, if in point of fact there is nothing that CAN reasonably be done that even has a chance of improving the situation, and the only actions that we COULD take would be only likely to make his situation worse and there is no realistic chance of their making his situation better, then we must not act.
Any thoughts on the assessment of these alternatives, especially any experience with the realities of intervention and the outcomes thereof, are greatly appreciated!