Sisters - i. e. half-sisters. Abigail and Zeruiah were daughters not of Jesse, but of a certain Nahash, whose widow Jesse took to wife 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 pre-eminent among the three 2 Samuel 2:13, 2 Samuel 2:16, Abishai was the eldest.