Current explanation of why close relatives could no longer marry by the time the Law was given:
"Reasoning from the Scriptures" page 253 Heading: Marriage
Why was brother-and-sister marriage not inappropriate at the beginning of mankind’s history? God created Adam and Eve perfect and purposed that all humankind descend from them. (Gen. 1:28; 3:20) Obviously some marrying of close relatives, especially within the first few generations, would occur. Even after sin made its appearance, there was relatively little danger of marked deformities in the children during early generations, because the human race was much closer to the perfection that had been enjoyed by Adam and Eve. This is attested to by the longevity of people then. (See Genesis 5:3-8; 25:7.) But about 2,500 years after Adam became a sinner, God prohibited incestuous marriage. This served to safeguard the offspring and it elevated the sexual morality of Jehovah’s servants above that of people around them who were then engaging in all manner of depraved practices.—See Leviticus 18:2-18.