Imamat 18:8

18:8 You must not have sexual intercourse with your father’s wife; she is your father’s nakedness. ¹ 

Ulangan 27:23

27:23 ‘Cursed is the one who has sexual relations with his mother-in-law.’ Then all the people will say, ‘Amen!’

[18:8]  ¹ tn Heb “the nakedness of your father she is.” See the note on v. 7 above. This law refers to another wife of the man’s father, who is not that man’s mother. The laws in the Pentateuch sometimes assume the possibility that a man may have more than one wife (cf., e.g., Deut 21:15-17).