Originally, Herod was married to the daughter of an Arabian King (Aretus). They probably had a peace treaty, because Aretus’ land bordered on Israel. However, Herod wanted to divorce his wife to marry Herodias. John repeatedly said that this was not lawful. The imperfect elegen means “repeatedly.” Thus Matthew 14:4 should be rendered, “For John had been repeatedly saying to him.” But why?
Herodias was Herod’s sister-in-law and niece. Carson writes, “Herodias was not only Antipas’s sister-in-law but also his niece, the daughter of his half-brother Aristobulus; but for most Jews there was no bar to marrying a niece.” This is why John was protesting their marriage so vigorously. To add to the disgusting nature of this love affair, Herodias’ daughter (his niece’s daughter!) was the one dancing seductively for Herod. This means that if Herod married Herodias, then Herod would have been getting turned on by his step-daughter. Moreover, Herodias was okay with her daughter seducing her uncle and prospective lover and husband, because verse 8 states that the dancer was “prompted by her mother…”).
This is the picture of a horrendously dysfunctional family!
 Carson, D.A. The Expositor’s Bible Commentary with the New Internation Version. Vol. 8. Grand Rapids, MI: Zondervan, 1984. 338.