BROTHER-SISTER MARRIAGE IN ANCIENT EGYPT
Egyptians often used the words brother and sister as terms of endearment when addressing their spouses, leading some early Nineteenth Century Egyptologists to speak of brother-sister marriages. We now know that while first cousin marriages were not uncommon, anything closer was as unacceptable then as it would be today, though men often married a deceased wife's sister.
The one exception to this rule was the King of Egypt who often married his sister. Early scholars were quite repulsed by this idea and developed the heiress theory by which inheritance passed through the female line. A male would become pharaoh but only by marrying the heiress princess who under normal circumstances was the potential king's sister. More and more Egyptologists are now discarding this idea forcing us to seek new reasons for the large number of brother-sister, father-daughter marriages.
Marriage between two families creates one much larger family which may have been an advantage to the general population but was a distinct disadvantage to royalty. Kings wanted an heir and a spare, but a family that expanded beyond that point may simply be creating potential rivals for the throne.
Marriage was the norm for Egyptians, but where does a princess find a husband of equal rank since marriage to a foreign prince was definitely not allowed. Brother-sister and even father-daughter marriages provided some solution to this problem.
It has also been suggested that brother-sister marriage was a way for Pharaohs to emulate the gods and goddesses and to set themselves apart from the rest of the population.
We in the Twentieth Century tend to think of kings as ordinary people with extra-ordinary jobs. The Ancient Egyptian believed their Pharaoh was a god and not at all like an ordinary man. Perhaps marriage to a sister was seen as a way of increasing the amount of royal blood in the next heir.