The answer depends on who you ask, or upon which OT commentary you rely:
"Ithra an Israelite - Or 'Jether the Ishmeelite' (1 Chron. 2:17). 'Ithra' and 'Jether' are practically the same names. 'Israelite' in the text is wrong. It should be either 'Ishmaelite' or 'Jezreelite' (2 Sam. 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 Chron. 2:16-17, is to suppose that Nahash was Jesse’s wife. If Zeruiah and Abigail were only sisters of David by the mother, then Nahash might be the name of her first husband." ~Albert Barnes' Notes on the Bible.
"... Nahash, sister to Zeruiah, Joab's mother ... was also called Jesse, David's father." ~Geneva Bible Translation Notes.
"... Jesse, whose name is here called Nahash, having two names, or this was his surname; though others think Nahash is the name of a woman, the wife of Jesse, and that these two sisters had two mothers, but one father; it seems by this expression, "went in to", as if Amasa was not begotten in the conjugal state, but was illegitimate." ~John Gill's Exposition of the Bible.
"Nahash — is thought by some to be another name of Jesse, or according to others, the name of Jesse’s wife." ~A Commentary on the Old and New Testaments by Robert Jamieson, A. R. Fausset and David Brown (JFB).
"The description given of Jithra as יִשְׂרְאֵלִי is very striking, since there was no reason whatever why it should be stated that Amasa's father was an Israelite... The true reading is evidently הַיּשְׁמְעֵאלִי, an Ishmaelite, according to 1 Chron. 2:17, where the name is written Jether, a contracted form of Jithra. From the description given of Abigail as a daughter of Nahash and sister of Zeruiah, not of David, some of the earlier commentators have very justly concluded that Abigail and Zeruiah were only step-sisters of David, i.e., daughters of his mother by Nahash and not by Jesse." ~Keil & Delitzsch Commentary on the Old Testament.
"Nahash is the name of Jesse'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." ~John Wesley's Notes on the Bible.
The foregoing are only some opinions related to 2 Sam. 17:25. There are others which I have not presented here.