Egyptian pharaohs used to marry their own sisters.
People practised many stupid things in the name of religion. The ancient Egyptian royalty practised inbreeding because of the mythology that all Pharaohs were the direct offspring of Ra, and diluting the Ra in their Pharaohs would be blasphemy.
Several communities in Asia and Africa marry their own first siblings, cousins, or uncles marry their nieces.
No large civilization has ever condoned incest as widely accepted practice.