Perashas Derachim

At the beginning of this week’s parshah Yaakov sent a message to Eisav. In the message were the words, “im Lavan garti – I lived with Lavan.” Rashi explains that Yaakov was informing Eisav that he had kept the entire Torah, as the word “garti” is the same numerical value as the amount of mitzvos in the Torah: 613. The following strong question concerning this statement has been discussed by the Rishonim and Acharonim: How could Yaakov say that he kept the entire Torah when he married two sisters, Rachel and Leah, which is biblically prohibited? Additionally, the Gemara in Yuma 28 says that Avraham Avinu kept the Torah; presumably the other avos did as well. How then did Yaakov marry two sisters?