John Gill Commentary Deuteronomy 27:23

"Cursed be he that lieth with his mother-in-law. And all the people shall say, Amen." — Deuteronomy 27:23 (ASV)

Cursed [be] he that lies with his mother-in-law. See (Leviticus 18:7, 18:8). All these incestuous or brutal copulations may well be supposed to be done in secret:

and all the people shall say Amen. as abhorring such incest.