I.G.
In most divorce cases there are two sides of the story and any outsider, even family members usually do not get to hear the complete picture.
I agree with the previous poster that your brother should get a divorce/ family law lawyer ASAP and follow his lawyers recommendations.
Other than that I would suggest that you and other family members stay out of it. Be supportive, but do not fan the fire with well meant advice, other than directing your brother to legal help.
It REALLY helps if at least one side stays calm and committed to resolve things in a reasonable manner.
From your post it sounds like something pretty serious has happened between them and you don't give us the complete picture. He should get the legal action going, usually the courts will set up a temporary arrangement for custody before the divorce is finalized, that will give them time to consider therapy and reconciliation.
It's really hard to say "how these cases usually go", because a lot depends on the circumstance and questions of jurisdiction. In WA family courts generally aim for joint custody arrangements.
Good luck.