[Graybeard58]

With the Egyptians and the Inca Royals where it was usually half brother marrying half sister

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Abraham and Sarah were half siblings.

[Harmless Teddy Bear]

Yes and they had fertility issues. Later their family did not continue the practice.

You can get away with it for one generation maybe but two is pushing your luck and at three you will definitely have problems.

With full siblings it happens even faster.