With our modern-day understanding of genetics, it is obvious that consanguinity (relation by blood) in marriage is problematic, since it greatly increases the likelihood that recessive traits will be expressed in the children. Even in ancient times, without the science of genetics, this must have been known. Therefore it may be considered a violation of natural law.
I think this idea might be extended to the marriage of adopted siblings. When they are brought up from early childhood in the same family, adopted siblings share the same psychological, moral, and cultural upbringing. To a large extent, they have the same strengths and the same weaknesses. When they marry, the weaknesses are propagated, and the manifestation of disorder is more likely in their children. Therefore one might think of it as moral and social inbreeding, and a violation of natural law.
There may be Bible passages to support this. The only one I can think of at the moment is Genesis 2:24:
That is why a man leaves his father and mother and clings to his wife, and the two of them become one body.
If a man marries his adopted sister, he has not really left his father and mother.
