The Hebrew that is translated as “medium” in English is translated in Basa as “the people of the grave.”
Many languages have terms for siblings that define whether one is younger or older in relation to another sibling.
Reiling / Swellengrebel (p. 178) say: “According to Josephus Herodias’ first husband, referred to in this verse, was Herod, son of Herod the Great and Mariamne (the second wife of that name). Herod the tetrarch was the son of Herod the Great and Malthake, whom he married after Mariamne. Hence ‘adelphou’ refers to an older brother of a different mother.”