It is wickedness, i.e. abominable and extraordinary wickedness, as the singularity of the punishment showeth. Both he and they; either, or both or all of them, if they consented to it.
Related Commentaries of Leviticus 20:14
Leviticus 20:14
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.