2 Kings 24:18 (ESV)

18 Zedekiah was twenty-one years old when he became king, and he reigned eleven years in Jerusalem. His mother’s name was Hamutal the daughter of Jeremiah of Libnah.

According to 1 Chronicles 3:15, Zedekiah was the third son of Josiah. After his younger brother Shallum and his older brother Jehoiakim, Zedekiah is the third heir to the throne. Initially, Zedekiah will have been loyal to the king of Babel (see Jeremiah 29:3 and Jeremiah 51:59).