How was it permissible for Abel (Habil) and Cain (Qabil) to marry their sisters?

Answer

In the name of Allah, it is not authentically established that Abel or Cain married. However, there is a narration indicating such a marriage after the killing of Abel and Cain, and Adam had other children. Allah has forbidden marriage to the sister, but if necessity demands, He can permit it for a brother. It is known that many things were previously permissible and later forbidden in the law of the Prophet (PBUH&HF), and the Qur’an indicates this. Allah is the Guide and Knower.