'Half sister' in the Bible

Three sons of Zeruiah [the half sister of David] were there: Joab, Abishai, and Asahel. Now Asahel was as light and swift-footed as one of the [wild] gazelles in the field.

Amnon was so frustrated because of his [half-] sister Tamar that he made himself sick, for she was a virgin, and Amnon thought it impossible for him to do anything to her.

Absalom put Amasa in command of the army instead of Joab. Now Amasa was the son of a man named Ithra the Israelite, who had married Abigail the daughter of Nahash, [the half sister of David and] the sister of Zeruiah, Joab’s mother.