Answer
When Allah Ta’ala sent Aadam and Hawa (Alayhis salaam) to the earth, the progeny of man began.
Haawa (Alayhis salaam) became pregnant twice with twins, one boy and one girl. Therefore, the two boys of both pregnancies married the alternative sisters. (Ma?ariful Qur’aan vol.3 pg.112; Darul Kitaab)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai