How was Zedekiah related to Nebuchadnezzar?

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He was Nebuchadnezzar's uncle.

And the king of Babylon made Mattariah his father's brother king in his stead, and changed his name to Zedekiah. 2 Kings 24:17

He was Nebuchadnezzar's brother.

And when the year was expired, king Nebuchadnezzar ... made Zedekiah his brother king over Judah and Jerusalem. 2 Chronicles 36:10