Shiva, the seven-day period of mourning, is observed upon the passing of the seven close relatives listed in Leviticus: father, mother, wife (or husband), son, daughter, brother, and sister.
But what about a half-sibling, is he or she considered to be a brother or a sister?
Based on the Talmud, the Code of Jewish Law states that one observes Shiva for half-siblings as well, provided that there is at least one common parent. (If they share a father then the obligation of mourning is biblically mandated. If they share only a common mother, the obligation is of rabbinic origin.
All the best,
Rabbi Baruch Davidson