Two women by this name are noted in the Old Testament.
1) The more famous of the two Abigails is the widow of Nabal of Carmel who later married David, with whom she bore a son called Kileab (2 Sam 3:3; in 1 Chron 3:1, he is called Daniel).
We first meet Abigail via her first husband, Nabal, a wealthy but also rather socially obtuse landlord from Carmel (not to be confused with Mount Carmel) in Judah; "she was an intelligent and beautiful woman, but her husband, a Calebite, was surly and mean in his dealings" (1 Sam 25:3). When David's men arrived to asked for provisions at a festive season, Nabal rebuffed them. Whatever we may think of David at this point in his life, he seemed to have understood (as did Nabal's servants, see v15f) his actions as an aid to Nabal and his properties and was, therefore, angry at the humiliation. Discovering what her husband had done, Abigail quickly remedied her husband's blunder by personally delivering "two hundred loaves of bread, two skins of wine, five dressed sheep, five seahs of roasted grain, a hundred cakes of raisins and two hundred cakes of pressed figs" (v18). David, not doubt, appreciated the woman's sensibilities but probably also her beauty, so that as soon as Nabal had died upon hearing what she had done, he sent for her hand in marriage (vv36-39). She and Ahinoam (David's other wife) kept with David all the years he spent on the run from Saul, which included a stint in Gath & Ziklag (1 Sam 27:3) and also being taken captives by, and rescued from, the Amalekites (1 Sam 29:5ff.). The last we hear of her she went up to Hebron with David where he was proclaimed king over Judah. We hear nothing of her after this.
2) The second Abigail in the Old Testament is, together Zeruiah, David's sister ( 1 Chron 2:16). The next verse asserts that she was married to "Jether the Ishmaelite" and she gave birth to a son Amasa (who later became Absalom's as well as David's commander).
2 Sam 17:25 sheds an interesting light, however, on Abigail's relationship to David, asserting that Abigail was "the daughter of Nahash and sister of Zeruiah the mother of Joab." That the same Abigail is clear from the reference to Zeruiah as her sister. The only way to explain Abigail's father being Nahash is to understand her as the child from her mother's earlier marriage (perhaps Nahash died young) before (re)marrying Jesse (David's father). If this is correct, then Abigail was, in fact, David's step-sister.
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