A.T. Robertson Commentary Mark 6:18

"For John said unto Herod, It is not lawful for thee to have thy brother`s wife." — Mark 6:18 (ASV)

Thy brother's wife (την γυναικα του αδελφου). While the brother was alive (Leviticus 18:16; Leviticus 20:21). After a brother's death it was often a duty to marry his widow.