“Then David came to Mahanaim, while Absalom crossed the Jordan with all the men of Israel. Now Absalom had set Amasa over the army instead of Joab. Amasa was the son of a man named Ithra the Ishmaelite, who had married Abigal daughter of Nahash, sister of Zeruiah, Joab’s mother. The Israelites and Absalom encamped in the land of Gilead.”
Amasa was the son of Ithra and Abigal, who was David’s other sister. So that Absalom, David’s son, and Amasa were first cousins, as was Joab. Remember that David came from a large family. They were set to do battle in Gilead.