'Half sister' in the Bible

Now David said, “Whoever strikes down a Jebusite first shall be chief and commander.” Joab the son of Zeruiah [David’s half sister] went up first, and so he was made chief.

Zelek the Ammonite, Naharai the Berothite, the armor bearer of Joab the son of Zeruiah [David’s half sister],

Joab the son of Zeruiah was in command of the army, and Jehoshaphat the son of Ahilud was the recorder;