Why was it not lawful for Herod Antipas to marry Herodias?

[tonychanyt]

Matthew 14:

3For Herod [Antipas] had seized John and bound him and put him in prison for the sake of Herodias, his brother Philip’s wife, 4because John had been saying to him, “It is not lawful for you to have her.”

Barnes explained:

This Herodias was a granddaughter of Herod the Great. She was first married to Herod Philip, by whom she had a daughter, Salome, probably the one that danced and pleased Herod. Josephus says that this marriage of Herod Antipas with Herodias took place while he was on a journey to Rome. He stopped at his brother's; fell in love with his wife; agreed to put away his own wife, the daughter of Aretas, King of Petraea; and Herodias agreed to leave her own husband and live with him. They were living, therefore, in adultery; and John, in faithfulness, though at the risk of his life, had reproved them for their crimes. Herod was guilty of two crimes in this act:

1. Of "adultery," since she was the wife of another man.
2. Of "incest," since she was a near relation, and such marriages were expressly forbidden, Leviticus 18:16.

Leviticus 18:

16 Do not have sexual relations with your brother’s wife; that would dishonor your brother.

Herod married Herodias, his brother's wife. That was forbidden.

But was the relationship incestuous?

Not exactly. It depends on your definition of incest. Herodias was also Herod's niece but it was not against Moses Law to marry one's niece.

Leviticus 18:

12 Do not have sexual relations with your father’s sister; she is your father’s close relative.

Herodias married her father's stepbrother which was not forbidden.