Zeruiah's three sons were Abishai, Joab, and Asahel. Abigail bore Amasa, whose father was Jether the Ishmaelite.
Absalom had installed Amasa in place of Joab over the army. (Amasa was the son of a man named Jether the Ishmaelite. His mother was Abigail, a daughter of Nahash and a sister of Zeruiah, Joab's mother.)
Then ask Amasa, "Aren't you my own flesh and blood? So may God deal with me, no matter how severely, if from this day forward you don't take Joab's place as commander of my army.'
Meanwhile, David ordered Amasa, "Muster the army of Judah here within three days, and be here yourself!"
"Furthermore, you're aware of what Zeruiah's son Joab did to me and to those two commanders of the armies of Israel, Ner's son Abner and Jether's son Amasa, whom he killed, and how he shed the blood of wartime during times of peace, staining the very belt he wears around his waist and the sandals he wears on his feet.
The LORD will repay him for his bloodshed because, without my father David's consent he attacked and murdered two men more righteous and better than he, Ner's son Abner, the commander of Israel's army and Jether's son Amasa, commander of Judah's army.