Whedon's Commentary on the Bible - Leviticus 20:14
14. Burnt with fire The heinousness of the incest specified in this verse is emphasized by burning the dead bodies of the culprits after they had been stoned. Cremation, so common among the Greeks and Romans, was exceedingly abhorrent to the Hebrews. Burning alive is not a penalty of the Mosaic law. See Joshua 7:15, note. Hence little confidence is to be put in the Targum of Palestine, which says, “Let them be burned with fire, with melted lead in their mouths.” read more
Wesley's Explanatory Notes - 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.They — All who consented to it. read more