So he took the letters, and went to Babylon and told this to all his brethren: The conjunction So is better rendered “Then” (Myers) or omitted (so Good News Bible). The pronouns he and his refer to Zerubbabel, which Good News Bible makes clear. He did not necessarily deliver the letters to everyone addressed, but probably took copies of the letters to the Jewish community in Babylon. His brethren refers to his fellow Jews. An alternative model for this verse is:

• Then Zerubbabel took copies of the king’s letters, and went to Babylon, where he told all his fellow Jews what had happened.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Esdras. (UBS Helps for Translators). Miami: UBS, 2019. For this and other handbooks for translators see here.