Leviticus 18:8
6 “‘No man is to approach any close relative to have sexual relations with her. I am the Lord. 7 You must not expose your father’s nakedness by having sexual relations with your mother. She is your mother; you must not have sexual relations with her. 8 You must not have sexual relations with your father’s wife; she is your father’s nakedness. 9 You must not have sexual relations with your sister, whether she is your father’s daughter or your mother’s daughter, whether she is born in the same household or born outside it; you must not have sexual relations with either of them. 10 You must not expose the nakedness of your son’s daughter or your daughter’s daughter by having sexual relations with them, because they are your own nakedness.
Amos 2:7
5 So I will set Judah on fire, and it will consume Jerusalem’s fortresses.” 6 This is what the Lord says: “Because Israel has committed three covenant transgressions—make that four—I will not revoke my decree of judgment. They sold the innocent for silver, the needy for a pair of sandals. 7 They trample on the dirt-covered heads of the poor; they push the destitute away. A man and his father go to the same girl; in this way they show disrespect for my moral purity. 8 They stretch out on clothing seized as collateral; they do so right beside every altar! They drink wine bought with the fines they have levied; they do so right in the temple of their God! 9 For Israel’s sake I destroyed the Amorites. They were as tall as cedars and as strong as oaks, but I destroyed the fruit on their branches and their roots in the ground.
Notes and References
"... Porter applies this law to a woman and her widowed mother-in-law, as in the case of Ruth and Naomi, but once Boaz marries Ruth, he is forbidden to Naomi. Hittite law permits cohabitation with free women who are daughters and mothers if they are in different lands but not if they are in the same place; but if they are slaves, cohabitation is permitted everywhere (HL §§ 191, 194; ANET 196). Greengus surmises that there was a comparable prohibition against father and son to have sexual relations with the same woman, a situation condemned in Amos 2:7, and can be inferred from HL § 194, which voids the penalty if the woman was a slave ..."
Milgrom, Jacob Leviticus 17-22: A New Translation with Introduction and Commentary (pp. 1547-1548) Doubleday, 2000