1. You can apply for divorce at any point of time. The pendency of 498A case has nothing to do with the merit of the divorce suit.
2. Your brother can leave country at any point of time. There is no such bar. The final decision of 498A case can be challenged by any party who is aggrieved by its verdict. The challenge of the decision of acquittal is not going to hamper career of any of the accused persons.