You shall not uncover the nakedness of your brother's wife; it is your brother's nakedness.
Leviticus 18:16
Surrounding context
18:15 You shall not uncover the nakedness of your daughter-in-law--she is your son's wife--you shall not uncover her nakedness.
18:16 You shall not uncover the nakedness of your brother's wife; it is your brother's nakedness.
18:17 You shall not uncover the nakedness of a woman and her daughter, nor shall you take her son's daughter or her daughter's daughter, to uncover her nakedness. They are near of kin to her. It is wickedness.