Imamat 18:8

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

Imamat 18:15

18:15 You must not have sexual intercourse with your daughter-in-law; she is your son’s wife. You must not have intercourse with her.

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).