They shall be burnt with fire - As there are worse crimes mentioned here, (see Leviticus 20:11 and Leviticus 20:17), where the delinquent is ordered simply to be put to death, or to be cut off, it is very likely that the crime mentioned in this verse was not punished by burning alive, but by some kind of branding, by which they were ever after rendered infamous. I need not add that the original, באש ישרפו baesh yishrephu, may, without violence to its grammatical meaning, be understood as above, though in other places it is certainly used to signify a consuming by fire. But the case in question requires some explanation; it is this: a man marries a wife, and afterward takes his mother-in-law or wife's mother to wife also: now for this offense the text says all three shall be burnt with fire, and this is understood as signifying that they shall be burnt alive. Now the first wife, we may safely presume, was completely innocent, and was legally married: for a man may take to wife the daughter if single, or the mother if a widow, and in neither of these cases can any blame attach to the man or the party he marries; the crime therefore lies in taking both. Either, therefore, they were all branded as infamous persons, and this certainly was severe enough in the case of the first wife; or the man and the woman taken last were burnt: but the text says, both he and they; therefore, we should seek for another interpretation of they shall be burnt with fire, than that which is commonly given. Branding with a hot iron would certainly accomplish every desirable end both for punishment and prevention of the crime; and because the Mosaic laws are so generally distinguished by humanity, it seems to be necessary to limit the meaning of the words as above.

a wife

Leviticus 18:17 You shall not uncover the nakedness of a woman and her daughter, neither shall you take her son's daughter, or her daughter's daughter...

Deuteronomy 27:23 Cursed be he that lies with his mother in law. And all the people shall say, Amen.

Amos 2:7 That pant after the dust of the earth on the head of the poor, and turn aside the way of the meek...

burnt

Leviticus 21:9 And the daughter of any priest, if she profane herself by playing the whore, she profanes her father: she shall be burnt with fire.

Joshua 7:15,25 And it shall be, that he that is taken with the accursed thing shall be burnt with fire, he and all that he has...

Self-Denial.
"If any man will come after Me, let him deny himself, and take up his cross, and follow Me."--Matt. xvi. 24. Good works are not the saint's sanctification, any more than drops of water are the fountain; but they spring as crystal drops from the fountain of sanctification. They are good, not when the saint intends them to be good, but when they conform to the divine law and proceed from a true faith. Yet the intention is of great importance; the Church has always taught that a work could not be called …
Abraham Kuyper—The Work of the Holy Spirit