How could the Sons of Jacob marry their sisters?
The Child’s Environment
אִישׁ אִישׁ אֶל כָּל שְׁאֵר בְּשָׂרוֹ לֹא תִקְרְבוּ לְגַלּוֹת עֶרְוָה אֲנִי ה’: (ויקרא יח:ו)
No man may come near to any of his close relatives to “uncover [their] nakedness.” Leviticus 18:6
The patriarchs – Abraham, Isaac, and Jacob – observed the Torah’s laws before they were formally given on Mount Sinai. Nonetheless, we have seen that Jacob’s sons married their sisters. The reason for this was because society was so morally depraved in the patriarchs’ times that there were simply no suitable choices for marriage partners outside the family circle. In those early days, it was crucial to preserve the spiritual and moral integrity of the family that was being groomed to become the Jewish people. Therefore, in order to ensure that their children not inherit negative traits from unworthy mothers or absorb negative attitudes while being raised by these women, Abraham’s male descendants were forced not to honor these prohibitions, which in any case would only become legally binding with the Giving of the Torah on Mount Sinai.
We learn from this how much care we should take to ensure that our children’s upbringing and environment support and encourage their absorption of the Torah’s teachings and values.1
Igrot Kodesh, vol. 20, pp. 262–263.