ISB ~ Mark 6 v18

your brother’s wife:John the Baptist was imprisoned and executed for publicly repudiating the illicit union of Herod Antipas and Herodias, the wife of his half-brother Philip. According to Lev 18:16 and 20:21, the Mosaic Law forbids the union of a man with his brother’s wife when the brother is still living. Since Philip was alive and well, the marriage between Antipas and Herodias was no marriage at all—it was adultery. See note on Mt 14:4.