Albert Barnes Commentary 1 Chronicles 2:16

"and their sisters were Zeruiah and Abigail. And the sons of Zeruiah: Abishai, and Joab, and Asahel, three." — 1 Chronicles 2:16 (ASV)

Sisters - that is, half-sisters. Abigail and Zeruiah were not daughters of Jesse, but of a certain Nahash, whose widow Jesse married (2 Samuel 17:25).

From the present passage, and from the fact that Abishai joined David as a comrade-in-arms before Joab (1 Samuel 26:6), it would seem that although Joab was preeminent among the three (2 Samuel 2:13, 16), Abishai was the eldest.