Why did God wait until after Abraham to prohibit close-relative marriage?
God did not prohibit close-relative marriage earlier because the early human genetic pool was still pure, so brothers marrying sisters and even Abraham marrying a half-sister produced no problems. Only after Abraham, as the genetic pool weakened and associated problems arose, did God place the taboo on close relatives.
Les teaches that from Adam and Eve came one race, and the question of where Cain got his wife shows that brothers married sisters for the first generation or two. That was no problem because the human race was so pure genetically. Even Abraham, 2,000 years after Adam, with God's blessing still married a half-sister, and there was nothing wrong with Isaac. God did not put a taboo on marrying close relatives until sometime even after Abraham, because by now the genetic pool was being weakened, and the resulting problems arise.