How could Yaakov marry sisters?
Tosfos ha'Shalem (2, and 29:24 (2), citing Sefer ha'Gan, Pane'ach Raza): For Nochrim, we follow the mother. Lavan's four daughters were from different mothers, so they were not considered sisters. 1
Moshav Zekenim #1: They converted, so they were not considered sisters. Mid'Rabanan one may not marry converted sisters, but only from the same mother, lest people say that they went to a lower Kedushah. 2
Moshav Zekenim #2: L'Chatchilah, the Avos observed the Torah, even though they were not commanded. Since he already was Mekadesh both, b'Di'eved, he did not want to divorce one. Also, he had Ru'ach ha'Kodesh that Rachel will not be buried with him (this implies that she will be his wife). 3
Moshav Zekenim #3, citing R. David: The Avos kept the Torah only in Eretz Yisrael. 4
Tosfos ha'Shalem: Even so, it was improper, and due to this, in the future Yaakov will disqualify himself from leading Birkas ha'Mazon (Pesachim 119b)
Hashem arranged that Yaakov marry sisters, so that when David's enemies maligned his lineage (for he comes from Rus), he could retort "look at your source - you come from sisters!'
This seems unlike the Midrash, "Im Lavan Garti - and I guarded all 613 Mitzvos" (Rashi to 32:5). Refer to 32:5:3:2.
What is the Maharal's approach, as to how Yaakov married two sisters (which the Torah would later prohibit)?
Gur Aryeh (to 46:10) #1: The Avos assumed the status of converts, who are like newly-born, and therefore have no prohibited relatives for marriage.
Gur Aryeh (to 46:10) #2: In the specific examples when the Avos contravened the Torah, they did so based on Ru'ach ha'Kodesh [and for specific reasons]. 1
Gur Aryeh (to 46:10) #3: When volunteering to fulfill a Mitzvah one has not been commanded, there is an intrinsic difference between Mitzvos Aseh and Mitzvos Lo Sa'aseh. For more regarding these three approaches, refer to 26:5:3.1.
Why does it say "Lo L'Ishah"?