The Torah is not in Heaven[1]
ויעש יעקב כן וימלא שבע זאת ויתן-לו את-רחל בתו לו לאשה
Yaakov [celebrated his marriage to Leah][2]. [When] the week was complete [Lavan] gave his daughter Rachel to [Yaakov] to be his wife[3]
After working seven years for Lavan for the right to marry his daughter Rachel, Yaakov was tricked. He thought he was being given Rachel as a bride, but after all was said and done he realized he had married Leah[4], Rachel’s sister. Lavan tried to justify his treachery, and concluded that Yaakov could marry Rachel as well once the week of celebrations ended. Yaakov did so, and thus was married to both sisters. Many authorities assume the Avos, the patriarchs, kept the entirety of the Torah before it was given[5]. This is based on various allusions to such an idea[6]. However, many struggle[7] to reconcile this with the fact that the Torah explicitly prohibits[8] a man from marrying two sisters. How then could Yaakov marry two sisters, which the Torah explains usually leads to strife?