Marriage of Adam’s Sons to his Daughters
It is a well known fact that legislations differ from one sharia to another, while the principles and beliefs remain the same in all of them. For example, making prostration of salutation was permitted in the Sharia of Yusuf (peace be upon him) but it is prohibited in ours. Also, war booty was prohibited for the nations before us but it is completely legal for us. The Qiblah of people before us used to be towards Bayt Al-Maqdis, but for us it is towards the Ka`bah.
In a similar way, marriage between brothers and sisters was permitted in the Sharia of Adam (peace be upon him) as opposed to those that came afterwards.
on this issue, Imam Ibn Katheer said:
Allah allowed Adam (peace be upon him) to marry his daughters to his sons for necessity… Al-Suddi narrated from Abu Maalik and Abu Salih, from Ibn `Abbas, from Murrah from Ibn Mas‘ood and from other companions of the Prophet (peace be upon him) that Adam did not have a baby boy unless it was accompanied by a girl, so he married the male of a couple to the female of another, and the female of a couple to the male of another.