If the law forbids marrying a sister or close relative, why did Abraham and Amram make such marriages?

These marriages were made long before the law that forbids them. Abraham lived centuries before Sinai, and Amram belonged to the generation before it: the law of Leviticus 18 was given through his son Moses. Leviticus presents these rules as a new standard that would set Israel apart from the customs of Egypt and Canaan: “After the doings of the land of Egypt, wherein ye dwelt, shall ye not do” (18:3). As Paul puts it, “where no law is, there is no transgression” (Romans 4:15). A law given at Sinai does not make sinners of people who lived before it was given.

Genesis does not hide these facts, and Moses did not hide his own descent; Gill remarks that Moses recorded his parents’ marriage though it might seem to reflect on his family. Nor does the blessing on Sarah approve the marriage as such. God blessed her as the mother of the promised son, by his own free promise. Genesis records much in Abraham’s life that it does not commend, including the half-truth “She is my sister”, by which he twice put Sarah at risk (Genesis 12:13; 20:2). The marriages and the later law belong to different stages of God’s dealings with his people, so they do not contradict each other.

Why the law came later

Some, such as Answers in Genesis, add that the first generations had to marry close relatives, and that the risk from such unions grew over time, so God forbade them when he gave the law. The law itself gives its reason as holiness and separation from the nations (Leviticus 18:3, 24-28).

A cousin, not an aunt?

The Hebrew dodah in Exodus 6:20 normally means ‘father’s sister’, and Numbers 26:59 calls Jochebed a daughter of Levi, which fits. But the Septuagint makes her ‘the daughter of his father’s brother’, a cousin; Gill reports the Vulgate and Syriac doing the same, and Adam Clarke preferred that reading.

A niece rather than a half-sister

Jewish tradition, followed by Gill, made Sarah a daughter of Abraham’s brother Haran, so that ‘the daughter of my father’ means a granddaughter of Terah. Most commentators, such as Keil and Delitzsch, take her as a true half-sister.