Here it says that that Tzidkiyah was Yehoyachin's uncle. In Divrei ha'Yamim II, 36:10 it says that he was his brother!
Radak (Divrei ha'Yamim II, 36:19): He was his uncle. [Sometimes, also other] relatives are called brothers, e.g. "Anashim Achim Anachnu", "Es Lot Achiv" (Bereishis 12:8, 14:16).
Malbim citing Mahari: Tzidkiyah was not Yoshiyah's son, rather, Yehoyakim's son 1 - "u'Vnei Yehoyakim Yechanyah 2 Veno Tzidkiyah Veno" (Divrei ha'Yamim I, 3:16). Tzidkiyah was two or three years older than his brother Yehoyachin, for the latter was 18 when he became king. Divrei ha'Yamim does not list them in order of birth, rather, in order of when they reigned. Shalum is Yeho'achaz; he was the youngest of Yoshiyah's sons, and he was 23 when he reigned. If so, Tzidkiyah (if he was Yoshiyah's son), who reigned 11 years later, was at least 34 or 35 at the time. Verse 18 says that he was 21 3 ! Rather, he was Yechanyah's brother; "Dodo" means his dear friend.
Malbim: Our verse discusses Yehoyachin's uncle. Indeed, Yehoyachin also had a brother Tzidkiyah (Divrei ha'Yamim I, 3:16). Nebuchadnetzar saw that Am ha'Aretz wanted to make the latter king in place of his father, so he changed Matanyah's name to Tzidkiyah, i.e. he stands in place of Tzidkiyah ben Yehoyakim, who claims that he should inherit kingship. Divrei ha'Yamim calls him his brother, i.e. he is in place of Yehoyachin's brother.
Malbim: This cannot be. Yeho'achaz and Tzidkiyah were brothers from the same mother, i.e. Chamutal bas Yirmeyah from Livnah (23:31, 24:18)! Indeed, Chazal (Vayikra Rabah 19:6) said that Yehoyakim had Bi'ah with his father's wife. Perhaps Tzidkiyah was born through this, and he is listed among Yoshiyah's sons and called Yehoyachin's uncle based on 'appearance' (Yoshiyah's wife gave birth to him), and he is listed among Yehoyakim's sons and called Yehoyachin's brother in truth! However, Tzidkiyah was a Tzadik; we should not say that he was a Mamzer. (Tanchuma (Vayetzei 4) learns verse 19 simply, that he was a Rasha. Indeed, Sanhedrin 103a says that he was a great Tzadik; the verse condemns him for not protesting. However, Rashi (Ta'anis 4a) says that a Mamzer cannot have Ru'ach ha'Kodesh, for Shechinah dwells only on those of proper lineage. This implies that he could be a Tzadik worthy of Ru'ach ha'Kodesh! - PF)
Yehoyachin is sometimes called Yechanyah. (PF)
Malbim: Do not conclude that Tzidkiyah was Yehoyachin's brother, rather, that Shalum is not Yeho'achaz, rather, another son! Chazal said that Yochanan is Yeho'achaz; he was born before Tzidkiyah.
Why did he change his name?
Why did he change his name to Tzidkiyah?
Rashi (from Horiyos 11b): Kah (Hashem) will be Matzdik (justify) the sentence if you rebel against me. 1
Malbim: Refer to 24:17:1:3.
Maharsha (11b): The sentence to exile the rest of Yisrael. Some Yisre'elim were already exiled, and Hashem would have exiled the rest of Yisrael if not for the merit of Tzidkiyahu.
