If a man marries both a woman and her mother, it is depraved. Both he and they must be burned in the fire, so that there will be no depravity among you.
Sins against the seventh commandment are here ordered to be severely punished. These are sins which, of all others, fools are most apt to make a mock at; but God would teach those the heinousness of the guilt by the extremity of the punishment that would not otherwise be taught it. I. Lying with another man's wife was made a capital crime.
Commenting on Leviticus 20:10-21
And if a man take a wife, and her mother,.... Marry both the one and the other, or commit uncleanness with them, they consenting to it: it is wickedness; abominable wickedness, shocking and detestable; there are other things, which also are wicked and not to be done, but this is extremely wicked, wickedness to a high degree: they shall be burnt with fire, both he...
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. (f) It is an abominable and detestable thing.