Matthew Poole and continuators on 2 Samuel 17:25

25 ¶ And Absalom made Amasa captain of the host instead of Joab: which Amasa was a mans son whose name was Ithra, an Israelite , that went in to Abigail the daughter of Nahash , sister to Zeruiah, Joabs mother.

Obj. He was an Ishmaelite, 1 Chron. 2. 17. Ans. Not Amasa; but Ithra, or Iether, Amasa's Father, is there so called, because he was such, either by his Birth from such Parents, or by his long habitation among them, or for some other reason now unknown. Compare 2 Sam. 15. 18. And Amasa is here called an Israelite, either, because he was a Proselyte: or, in opposition to Ioab, who was of the Tribe of Iudah, as Amasa was of one of the ten Tribes: or rather, to intimate, that although he or his Parents were called Ishmaelites for some reason, yet as to their extraction they were indeed Israelites. Which if Amasa had not been, it is not probable that he could have had so powerful an influence upon the Tribe of Iudah, as he had, chap. 19. 14.

i. e. Lay with her, whether being first Married to her, or not, is uncertain.

Nahash is either another name of Iesse: or rather, the name of Iesse's Wife, by whom he had this Abigail, as he had Zeruiah by another Wife; so they were sisters by the Father, but not by the Mother; and Nahash is here named to signify so much.