Marriage to a half-sibling – this was practiced in Egypt. Is there evidence that this was a ANE general practice? Remembering the culture such a marriage is not hard to understand.
This practice wasn’t banned until the Law was given, however, if the clean/unclean animals law was known was this one of those things that would have been known but God did not deal with because there was more important things He was dealing with in Abraham?
Note in later generations the wives were not actually half-siblings but cousins, and God promoted marriage within the tribes and ordered the tribes not to marry outside.
An interesting feature is that if there was not health problems caused by inter-marriage through the generations, perhaps the DNA of the near relatives wasn’t so corrupt or perhaps the generational inter-marriage where the parents of the couple were already half-siblings was not encouraged because the problems were evident.