Albert Barnes Commentary 2 Chronicles 36:10

"And at the return of the year king Nebuchadnezzar sent, and brought him to Babylon, with the goodly vessels of the house of Jehovah, and made Zedekiah his brother king over Judah and Jerusalem." — 2 Chronicles 36:10 (ASV)

When the year was expired - This phrase literally means “at the return of the year,” that is, at the start of the season for military expeditions. The expedition against Jehoiakim probably took place late in the autumn of one year, and the one against Jehoiachin early in the spring of the next.

Strictly speaking, Zedekiah was Jehoiachin’s uncle, being the youngest of the sons of Josiah. He was nearly the same age as Jehoiachin and is called his “brother” here .