Leviticus 18:17
Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son’s daughter, or her daughter’s daughter, to uncover her nakedness; for they are her near kinswomen: it is wickedness. Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son’s daughter, or her daughter’s daughter, to uncover her nakedness; for they are her near kinswomen: it is wickedness.
The law forbids a man to have sexual relations with both a woman and her daughter. The prohibition is not limited to the daughter but extends to the next generation on either side. It includes both her son's daughter and her daughter's daughter, so that neither granddaughter may be taken for the same purpose.
The stated reason is that these women are her near kinswomen. The command therefore sets a firm boundary against sexual relations within close family ties. Such conduct is named as wickedness. It belongs to the Lord's rules for preserving moral purity.
Its principle condemns forbidden sexual familiarity as a violation of that purity. It likewise condemns practices that corrupt young women. Those practices are understood as doing them harm.