It has not stated in the main authentic Shia sources that Abraham (AS) and Sara were brother and sister to each other. Indeed, it has been reported in some Sunni sources and then in some Shia books that when Nimrud threw Abraham into the fire due to the latter breaking and insulting the idols, Almighty Allah made the fire cold, safe and secure for the prophet enabling him to come out of the fire unhurt.[1] Nimrud was struck with fear seeing this incident. That was why he confiscated his property and ordered him to be banished from the land under his rule.  Prophet Abraham then said: "Return to me my property and I will leave your land."
Nimrud opposed him and Nimrud's judge said: "If this main remains in this land, he will destroy your religion; it is therefore better that you give him back his property and let him leave the land."
Nimrud ordered that Abraham's properties and belongings be returned to him.  There and then Abraham put his wife, Sara, who was the daughter of his mother's sister into a box to protect her against the gaze of Nimrud's followers. Then he and Lot who was the son of his mother's sister left the land under Nimrud's rule. When he was leaving Nimrud's land, he came across Nimrud's men who each took one tenth of their properties until one of them walked over to the box and said: "Open that case, so we can see what is in there."
Abraham said: Whatever you think is in the box I will give you one tenth of it but do not open it; the custom's officer and Numrid's agent did not accept and Abraham was forced to open the box. When he saw Sara's beautiful face, he was amazed and then he inquired: "Who is this woman with you?"
Abraham (AS) dissimulated (did tawriyah) and said: "She is my sister." He meant his sister in faith, not his real sister.[2] The reason he dissimulated was to protect Sara; because if he had said that she was his wife, they would kill her.[3]
Therefore, Prophet Abraham and Sara were not real brother and sister so we cannot conclude that incestuous marriages (i.e. marriage of a man to his sister, niece and aunt) were allowed in other religions or at least until the time of Prophet Abraham, peace be upon him. It has been said only in regards to Adam's children, based on a report, that since incestuous marriage was not prohibited in the time of Prophet Adam and there was no way for the man's offspring to survive, therefore Cain and Abel married their twin sisters. It was after these marriages that God forbade marriage of a man to his sister.[4]

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