The time arrived when the two young men desired life partners. This was part of Allah's plan for mankind, to multiply and form nations with different cultures and colors. Allah revealed to Adam that he should marry each son to the twin sister of the other. Adam instructed his children according to Allah's command, but Cain was displeased with the partner chosen for him, for Abel's twin sister was not as beautiful as his own.
Prophet Adam
By MuslimYouth
Stories of the Prophets Written by Al-Imam ibn Kathir Translated by Muhammad Mustapha Geme'ah, Al-Azhar More