The answer
Yes. ’uring the time of the first generation, marriage amongst brothers and sisters was permitted for the purpose of procreation. But gradually, it became forbidden.
H.G. Bishop Moussa
H.G. Bishop Moussa · Youth Concerns · 5
Faith Q&A › Youth Concerns › The Bible
Is it true that Cain and Abel married their sisters?
H.G. Bishop Moussa, Bishop of Youth, Youth Concerns (Questions & Answers). Compiled by Lisa Agaybi. Coptic Orthodox Patriarchate, Bishopric of Youth Bookstore.