Is a divorce issued by a husband valid if he was in a state of anger, knowing that he is taking medication that causes him to be irritable?

Anger does not prevent a divorce from occurring as long as the person is aware of what he is saying and is not overcome by his intellect. If your brother pronounced the divorce while conscious and not deprived of his awareness, then the divorce has occurred. He can reconcile with his wife before the expiry of her 'iddah (waiting period) if he has not completed three divorces. However, if this divorce completes the three, then she is irrevocably divorced from him, and she is not lawful for him until she marries another man, and he consummates the marriage with her, and then divorces her or dies, and her 'iddah ends.