Answer 34: We cannot give a precise and clear-cut answer to this question. The Holy Quran says in the first verse of chapter al-Nisa, O people! Fear your Lord Who created you from a single soul and made its mate from within it, and from that pair spread many men and women and fear Allah in Whose name you ask for (your rights) and pay attention to the ties of relationship." Some mufassereen (exponents) of the Holy Quran use the verse "and from that pair spread many men and women" to conclude that the Adam's offspring and the existing human generation are from Adam and Eve. They say that no third individual were involved in the spread of their generation, otherwise, the verse would have been "and from that pair and other being spread many men and women". They therefore say that marriage between brothers and sisters took place among Adam's children because it was permitted in the Shari'ah of Adam (peace be upon him) as opposed to those that came afterwards. Rationally, there is no objection to such a marriage, if it had been permissible at a time. If someone says that marriage between brothers and sister had been forbidden even in other religions, the answer is this that the existence of such a commandment does not necessarily mean that injunction had been eternal and permanent because it is legislative (tashri'ei) and changeable. There would be no objection to the marriage between brothers and sisters, if it had been out of necessity in a particular part of the history of human generation. As understood from the narratives, haram things become halal for necessity.
According to some narrations in Behar al-Anwar (Vol.11 pg. 233), marriage between brothers and sisters had been forbidden in all Shariah and divine books. They say that Adam and Eve had as many as seventy sons and daughters. When Cain (Qabil) killed Habil (Abel), Adam cried in grief. Thereupon, God granted him a male human being without a female partner. He was called "Hebatullah" and then a horri was sent down and was called Barakah. Adam was ordered to marry the hori to Seth. Also, another hori was sent down and she was married to Adam's another son called Yafeth. God granted Seth a son and Yafeth a daughter who were married to each other. The prophets and messengers' offspring are from this couple.
There is a set of other narrations from which it is understood that God granted Adam a male and a female child with Eve's each delivery. The male child married the female child and it is stated in the traditions that the prohibition of marriage between brothers and sisters was not permanent. Although Eve was created from Adam's ribs, she was made halal for him. (Allamah Baqir Majlisi have taken these traditions for taqiyah (dissimulation)).
There is another possibility in this regard which is inferred from the traditions. The strongest possibility which is consistent with the apparent meaning of the Quranic verses is the first possibility in this answer. God knows best.