Did brothers marry sisters among the children of Adam?
Question

I am asking this question on behalf of my paternal aunt, who is troubled by this matter. It has to do with the beginning of humankind. The Qur’an clearly speaks about Allah’s Prophet Adam and the reason why he came down to earth, but there is no clear statement that Adam and Hawwa’ were the only parents of humanity. In other words, if they were the only two people on earth, and they had children, then how did humankind increase in numbers after them? In other words, was it permissible at that time for sisters and brothers to marry one another, and to have children from one another?
Praise be to Allah.Allah tells us that all humans are descended from Adam and his wife. Allah, may He be exalted, says (interpretation of the meaning):
{O mankind, fear your Lord, who created you from one soul and created from it its mate and dispersed from both of them many men and women} [an-Nisa’ 4:1]
{It is He who created you from one soul and created from it its mate that he might dwell in security with her. And when he covers her, she carries a light burden and continues therein} [al-A‘raf 7:189].
This clearly indicates that Adam is the father of humanity, and that he is the one from whose progeny all humankind were created, and that all humans on earth go back to Adam and his wife.
In some hadiths, it says that Adam and his wife bore children, a male and female (twins) in each pregnancy. When they grew up, the male from one pregnancy would marry the female from the pregnancy that came before him, and the female from the second pregnancy would marry the male from the first pregnancy. That was permissible, even though they were siblings from the same father and mother, as it was a case of necessity. After their numbers increased, it was forbidden for a brother to marry his sister, even if she resulted from a different pregnancy [and was not his twin].
And Allah knows best.