|ו וַיַּעַן דָּוִד וַיֹּאמֶר אֶל-אֲחִימֶלֶךְ הַחִתִּי, וְאֶל-אֲבִישַׁי בֶּן-צְרוּיָה אֲחִי יוֹאָב לֵאמֹר, מִי-יֵרֵד אִתִּי אֶל-שָׁאוּל, אֶל-הַמַּחֲנֶה; וַיֹּאמֶר אֲבִישַׁי, אֲנִי אֵרֵד עִמָּךְ.||6 Then answered David and said to Ahimelech the Hittite, and to Abishai the son of Zeruiah, brother to Joab, saying: 'Who will go down with me to Saul to the camp?' And Abishai said: 'I will go down with thee.'|
The question is: Who is this Tzeruya who is the mother of Yoav, Avishai and also Asael?
So we look into it, and we find two places where she is mentioned:
1) II Samuel 17: 25
|כה וְאֶת-עֲמָשָׂא, שָׂם אַבְשָׁלֹם תַּחַת יוֹאָב--עַל-הַצָּבָא; וַעֲמָשָׂא בֶן-אִישׁ, וּשְׁמוֹ יִתְרָא הַיִּשְׂרְאֵלִי, אֲשֶׁר-בָּא אֶל-אֲבִיגַל בַּת-נָחָשׁ, אֲחוֹת צְרוּיָה אֵם יוֹאָב.||25 And Absalom had set Amasa over the host instead of Joab. Now Amasa was the son of a man, whose name was Ithra the Jesraelite, that went in to Abigal the daughter of Nahash, sister to Zeruiah Joab's mother.|
2) 1 Chronicles 2: 16-
|טז וְאַחְיֹתֵיהֶם, צְרוּיָה וַאֲבִיגָיִל; וּבְנֵי צְרוּיָה, אַבְשַׁי וְיוֹאָב וַעֲשָׂהאֵל--שְׁלֹשָׁה.||16 And their sisters were Zeruiah and Abigail. And the sons of Zeruiah: Abishai, and Joab, and Asahel, three.|
The problem is that in Samuel, Abigail is identified as the daughter of Nachash and the sister of Tzeruya, whereas in Chronicles, they are just mentioned along with all the other children of Jesse. So who is Tzeruya, and is she the daughter of Nachash or of Jesse?
The traditional commentaries in the Book of Samuel simply say that Nachash and Jesse were both names for the same person: Jesse, father of David. They explain that Jesse never sinned, and only died because of the sin with the serpent in Gan Eden.
Modern scholars can and do suggest that the insertion of the word 'Nachash' is a typo or scribal error, and that it should have said Jesse.
But there's an interesting Malbim to Divrei Hayamim I found today that sheds some light on this as well:
טו) דויד השביעי. וזה סותר למה שבשמואל (א' טז, י; יז, יב) חושב לישי שמונה בנים ודוד הוא הקטן. ודעתחז"ל במדרש שהשמיני הוא אליהו הנזכר בין הנגידים שהקים דוד על יהודה, אליהו אחי דוד (לקמן כז, יח), והואהיה קטן מדוד, וקרא לדוד קטן על שהיה מקטין א"ע, וכן פרש"י. וראב"ע ורד"ק פירשו שאליהו היה אח מן האבולא מן האם, ולכן לא חשבו פה מפני שאומר ואחיותיהם צרויה ואביגיל, והם היו אחיות מן האם ולא מן האב,וא"כ לא היו אחיות לאליהו לא מן האב ולא מן האם, וכדבריהם נראה ממ"ש (ש"ב יז, כה) ועמשא בן איש ושמויתרא הישראלי אשר בא אל אביגיל בת נחש אחות צרויה אם יואב, מבואר (לפי הפשט, והדרש ידוע) שאביגילהיתה בת נחש לא בת ישי, וא"כ היתה אחותם רק מן האם, לכן אמר אחות צרויה ולא אמר אחות דוד, מפנישאחות צרויה היתה מן האב ומן האם שגם צרויה היתה בת נחש, ואמם נשאת לישי באלמנותה:
According to this explanation, Tzeruya and Avigail are half-sisters of David. They are the children of his mother who is given to Nachash when she is widowed.
The problem is: Nachash doesn't seem to be dead. And he would need to be dead for Tzeruya to be a widow.
Yes, Saul did fight against him in the battle to defend the men of Yavesh-Gilad. But he doesn't seem to die there, because he crops up again. See 2 Samuel 10:
If Nachash was alive in the time of David, then he didn't die in the time of Saul.
So there is a possibility that Nachash is the name of all of the kings of Ammon (a title like Pharoah), but if so, then Hanun should also be called Nachash.
Enter Josephus, who confirms that the first Nachash did die in the battle of Saul (though we don't know his sources) and thus whoever this Nachash is, he's some other Nachash.
See Josephus, Volume 6, Book 5, Entry 3:
3. So being desirous to turn the people to this war against the Ammonites by fear of the losses they should otherwise undergo, and that they might the more suddenly be gathered together, he cut the sinews of his oxen, and threatened to do the same to all such as did not come with their armor to Jordan the next day, and follow him and Samuel the prophet whithersoever they should lead them. So they came together, out of fear of the losses they were threatened with, at the appointed time. And the multitude were numbered at the city Bezek. And he found the number of those that were gathered together, besides that of the tribe of Judah, to be seven hundred thousand, while those of that tribe were seventy thousand. So he passed over Jordan, and proceeded in marching all that night, thirty furlongs, and came to Jabesh before sun-rising. So he divided the army into three companies; and fell upon their enemies on every side on the sudden, and when they expected no such thing; and joining battle with them, they slew a great many of the Ammonites, as also their king Nabash. This glorious action was done by Saul, and was related with great commendation of him to all the Hebrews; and he thence gained a wonderful reputation for his valor: for although there were some of them that contemned him before, they now changed their minds, and honored him, and esteemed him as the best of men: for he did not content himself with having saved the inhabitants of Jabesh only, but he made an expedition into the country of the Ammonites, and laid it all waste, and took a large prey, and so returned to his own country most gloriously. So the people were greatly pleased at these excellent performances of Saul, and rejoiced that they had constituted him their king. They also made a clamor against those that pretended he would be of no advantage to their affairs; and they said, Where now are these men? - let them be brought to punishment, with all the like things that multitudes usually say when they are elevated with prosperity, against those that lately had despised the authors of it. But Saul, although he took the good-will and the affection of these men very kindly, yet did he swear that he would not see any of his countrymen slain that day, since it was absurd to mix this victory, which God had given them, with the blood and slaughter of those that were of the same lineage with themselves; and that it was more agreeable to be men of a friendly disposition, and so to betake themselves to feasting.This is still not super-satisfying, because who exactly is this second Nachash, and why does he succeed the throne without us knowing? A scribal error would be the most logical response to this, but I don't know where I can find proof of this. For now, we will go with Tzeruya being the daughter of Nachash of Ammon and half-sister of David.