MY BROTHER HIT HIS WIFE

Depending on the circumstances, an arrest warrant may or may not go out.  The police could simply take a report and forward it to the local prosecutor or may do nothing at all.  If your brother's wife had no visible injury and/or hit him first, the police might decide that your brother was acting in self-defense.  That he left before the police got there really doesn't mean anything- leaving is often the best idea to diffuse a volatile situation.  They may advise his wife to seek a restraining order but she is not legally required to do so.   

There's a very good chance that your brother's wife will get a court order keeping him out of the house and away from her, because he's committed an act of domestic violence.  He'll be served with a temporary order, first, and a notice for the court date.  He can go to court and contest the charge.  If the court finds in favor of the wife, a permanent restraining order will get issued.

Your brother needs a lawyer.