How is your brother doing, Worriedsister?
I see you didn't get any responses at the time you posted.
That may be because the possibilities in this case may be too wide and varied.
Basically I would guess that all you can do is remain a friend to your brother, and hope that he begins to see the contrast between the marriage he has, and the marriage he wants.
I suppose one thing you could all do, is, when he tells one of you something, ask if the children saw the incident...or some other question that you all feel is appropriate.
The idea being to plant a seed of awareness...but not embarrass or otherwise alienate your brother. He sounds like a decent man trying to be loyal to his marriage commitment.
His tender feelings are exposed when he is asking for another to listen.
Hope everything turned out well for all of you.
My best,
Fledgling