20:12 my sister. Sarah was Abraham’s half-sister. In the early centuries after the Dispersion, close marriages were often necessary, with very small tribal populations. This may have been especially desirable in godly families, in order to preserve faithfulness to God’s revelation and His purposes. As noted before, this situation was not harmful genetically until many harmful mutations could have accumulated in the nation’s genetic pool. By the time of Moses, this had apparently become a problem, and laws against incest were established.