Albert Barnes Commentary 2 Samuel 17:25

"And Absalom set Amasa over the host instead of Joab. Now Amasa was the son of a man, whose name was Ithra the Israelite, that went in to Abigal the daughter of Nahash, sister to Zeruiah, Joab`s mother." — 2 Samuel 17:25 (ASV)

Ithra an Israelite — Or “Jether the Ishmeelite” (1 Chronicles 2:17). “Ithra” and “Jether” are practically the same names. The word “Israelite” in the text is incorrect; it should be either “Ishmaelite” or “Jezreelite” (2 Samuel 3:2).

Abigail the daughter of Nahash — If Zeruiah and Abigail were Jesse’s daughters, the only probable way of reconciling our text with 1 Chronicles 2:16–17 is to suppose that Nahash was Jesse’s wife. If Zeruiah and Abigail were only sisters of David on their mother’s side, then Nahash might have been the name of her first husband.