A casigamy marriage is a marriage between a brother and his sister.
Casigamy marriages were common in some primitive tribes and cultures, including in Ancient Rome and among the royal families of Ancient Egypt. However, relationships between all family members, including siblings, are not supported by most modern cultures. In most contemporary societies, casigamy marriages are now illegal.
Casigamy marriages were often a matter of practicality in the ancient societies that practiced them. Many primitive tribes had a limited number of eligible partners, so if love blossomed between siblings it was not condemned.
Royals with casigamy marriages likely married their siblings to keep the royal family small. By limiting the number of royals, there was less competition for the throne and the family fortune. Like the primitive tribes who practiced casigamy marriages, royals who married their siblings may also have done so because there were a limited number of eligible partners of their social standing.
Casigamy marriages are not supported by modern society. Any children from such a union have a significantly higher risk of genetic disorders. Casigamy marriages are also seen as morally wrong.
