Is informing my friend's brother of the secret I swore to keep – after he gave me permission – considered a divorce, and what must I do?

You do not break your oath if you inform the brother of the one who entrusted you with a secret and made you swear to keep it, as long as the one who made you swear has agreed to your informing his elder brother. The reason for which you swore has ceased to exist, but only with respect to the brother of the one who made you swear.