Leviticus 20:19
And thou shalt not uncover the nakedness of thy mother’s sister, nor of thy father’s sister: for he uncovereth his near kin: they shall bear their iniquity. And thou shalt not uncover the nakedness of thy mother’s sister, nor of thy father’s sister: for he uncovereth his near kin: they shall bear their iniquity.
Sexual relations with a mother's sister or a father's sister are forbidden as uncovering one who is near of kin, and those who do so bear guilt for their iniquity. The chapter belongs among those that set out the degrees of kinship prohibited for marriage. It gives no prohibition against marrying cousins.
The prohibitions on sexual relationships in Leviticus 20:10-21 remain in force because they rest on moral considerations. Marriage to an uncle's widow belongs to those enduring moral rules, and such a union is also wrong because it confuses family relationships and spiritual discernment.
The law stands as part of the warning against the practices that marked the nations living in Canaan, exposing their evil character and the evil those practices brought upon those nations. It is one of the laws in Leviticus given by God as an object-lesson that emphasizes the exceeding sinfulness of sin. The laws prohibiting unions within forbidden degrees of kinship and affinity help explain why Deuteronomy 23:2 may refer especially to the offspring of incestuous unions or marriages violating rules of affinity.