C. F. Keil and Franz Delitzsch on Leviticus 18:9

Lev 18:9

By the sister, the daughter of father or mother, we are to understand only the step-or half-sister, who had either the same father or the same mother as the brother had. The clause, “whether born at home or born abroad,” does not refer to legitimate or illegitimate birth, but is to be taken as a more precise definition of the words, daughter of thy father or of thy mother, and understood, as Lud. de Dieu supposes, as referring to the half-sister “of the first marriage, whether the father’s daughter left by a deceased wife, or the mother’s daughter left by a deceased husband,” so that the person marrying her would be a son by a second marriage. Sexual intercourse with a half-sister is described as חסד in Lev 20:17, and threatened with extermination. This word generally signifies sparing love, favour, grace; but here, as in Pro 14:34, it means dishonour, shame, from the Piel חסּד, to dishonour.