Leviticus 18:17-19

Contemporary English Version

^17 (A) And don't have sex with the daughter or granddaughter of any woman that you have earlier had sex with. You will be having sex with her closest relatives, and that would make you unclean. ¹⁸ As long as your wife is alive, don't cause trouble for her by taking one of her sisters as a second wife.

^19 (B) When a woman is having her monthly period, she is unclean, so don't have sex with her.