Leviticus 20:14
And if a man take a wife and her mother, it is wickedness: they shall be burnt with fire, both he and they; that there be no wickedness among you. And if a man take a wife and her mother, it is wickedness: they shall be burnt with fire, both he and they; that there be no wickedness among you.
Leviticus 20:14 stands in chapters 18-20, within the larger block of chapters 5-23 dealing largely with laws of sacrifices and offerings, consecration and duties of priests, clean and unclean animals, leprosy and unsanitary houses, and the feasts of the year. These are laws God gave to Israel in contrast to the customs of the surrounding nations. The nations the Lord cast out of the Holy Land before Israel had given themselves up to pagan abominations with Satan ruling through the flesh, and after four hundred years of mercy in which God sent His servants to sojourn among them to win them to the right way they filled up their cup of iniquity, so blotting them from the earth was a blessing to future generations.
The verse identifies degrees of kinship in which marriage is prohibited, forbidding a man to take both a woman and her mother as wives as a union violating the Mosaic laws governing forbidden family relationships. It gives no prohibition against cousins marrying. The prohibition on this sexual relationship remains in force because it is based on moral considerations. Its prohibitions and penalties are among the object lessons teaching the exceeding sinfulness of sin and showing sin's gravity.
Taking a wife and her mother is wickedness to be put away by burning with fire both the man and both women, so that no wickedness remains among the people. The severe punishment reflects the strong penalties for social crimes in the ancient world, and the Mosaic code was better than other contemporary codes. Because Israel was then a theocracy with God as its direct ruler, severe punishment was expected for heinous crimes of this kind, and the penalty does not support the inquisition.
The burning commanded here identifies the valley of Hinnom, the place associated with Gehenna, as the historical and literal place of punishment by burning. This burning, together with the destruction of Moloch worshipers commanded in the same chapter and the burning commanded for a priest's daughter who profaned herself, was carried out where the image stood in the valley.