According to those that say the Avos kept the Torah beforehand (here), how was Yaakov allowed to marry two sisters?
(Yes, I'm aware that there are many answers)
Please see Rav Moshe Feinstein's answer here (linked in the other thread).
He says the only reason one may not marry two sisters is the prohibition against קידושין (the preliminary stage of marriage) with two sisters and that קידושין did not apply to non-recipients of the תורה (and still doesn't).
The Lubavitcher Rebbe zt"l answers that the Avos' observance of the entire Torah was a personal stringency, which could not override societal norms that were generally accepted by the descendants of Noach. One of these was not to deceive each other (hence Yaakov could challenge Lavan, "Why did you deceive me?" and Lavan had to find an excuse - Gen. 29:25-26). Since he had promised Rachel that he would marry her, then he was obligated to keep his word, even though circumstances had changed.
The Michtav MiEliyahu brings a very profound answer. The purpose of Torah is to connect to God. We say that Avos kept the entire Torah yet this is simply not possible. There are many sacrifices they never offered, they did not steal anything so they could engage in the mitzvah of restituting theft, etc. Therefore this must mean that through their exceptionally high level of spirituality they were able to achieve a dveikus with God that someone who kept the whole Torah as we understand it did. At this level Yaakov Avinu, a"h, recognized that it was part of God's plan for history that the 12 tribes come from these 4 particular women. Therefore even though for us such a marriage is forbidden for him it was meritorious because of the ultimate outcome. However, the MM notes that as soon as the 12 tribes were done being born Rachel died. And there was no delay. She died giving birth to Binyamin because once they were all there the special dispensation to marry two sisters no longer applied.
I once heard 'outside' a very interesting take on Yaakov marrying two sisters.
As the other answers have mentioned, the avos kept halacha, but voluntarily.
Yaakov loved Rachel, and Rachel loved Yaakov. One can only imagine how much flirting and starry-eyed glances transpired between Yaakov and Rachel, during the seven years that Yaakov worked for Lavan for the privilege of marrying Rachel.
(As an aside, no one need be concerned that it shows lack of respect for the avos to say that Yaakov flirted. He was engaged - he was supposed to look at his betrothed and feel a desire for her!)
So after seven years of yearning for each other, the wedding day comes. Yaakov wakes up in the tent in the morning, and behold! It was Leah!
Rachel has been anxiously awaiting the love of her life for seven years. She would be absolutely devastated if she couldn't marry Yaakov.
If the Torah had already been given, Yaakov wouldn't have any options. (Even divorcing Leah wouldn't help, because the aveirah is to marry two sisters, even one at a time, while the other is still living).
However, because it would hurt Rachel (and Yaakov) so much if they couldn't be together, Yaakov chose to set aside his voluntary observance for her.
This teaches us an important lesson.
Halacha is halacha. We must never compromise our observance of it.
However, sometimes Jews will keep a chumra, at the expense of someone else's feelings.
That is not correct. Another's feelings are more important than your unnecessary stringencies.
Or, as one of my teachers would say "don't be frum on the other guy's cheshbon".
Cut and Paste from an answer I gave at the other thread:
With regards to the questions and contradictions between the laws in the Torah and the actions of the Avot, R' Chaim Volozhin, in Nefesh Hachaim Sha'ar 1 Chapter 21, says that when our sages say the Avot kept the whole Torah (in this he included Amram as well, not just Avraham, Yitzchok and Yaakov), they didn't do so because they were commanded to do so. If they were commanded to do so, they would never violate any of the commandments of Torah. Rather they understood with the great Tikkunim that are accomplished by doing the Mitzvos, and the destruction the would be caused by them not fulfilling the Mitzvos. So they always made sure to do the actions that they understood that their souls would cause Tikkunim.
Therefore, when for example, Ya'akov Avinu saw that his soul could accomplish amazing Tikkunim in this world by marrying two sisters (since he understood that it was specifically through them that the Jewish nation would be brought into this world), he worked hard to make sure this happened. So too, Amram married his aunt so since he understood that doing this would bring Moshe Rabbeinu into this world.
And that's one of the reasons why the Torah wasn't given to the Avos, since if they already had the Torah, it wouldn't matter if they realized that great Tikkunim could be accomplished by going against the Torah, they would not be able to do so.
Ger shnisgayeir kekatan shenolad dami (Yeb. 97b). (A convert is like a newly-born baby.)
This means that when somebody converts, it is as if they are born anew, and that their biological parents are not their halakhic parents. This means that they were not sisters, and there would not be a problem in the first place for Jacob to be married to both of them. IIRC, I heard this from one of the Roshei Yeshivah of RIETS.
A practical ramification of this, I believe, is that converts are not obligated in Kibbud Horim to the same extent that people born initially Jewish are.
With regards to the issue of Yaakov marrying two sisters, you should know that the Torah rests on three pillars - time, place and items. Time - not on all days is doing work forbidden like Shabbos and Yom Tov, or is eating chametz forbidden like Pesach, or are obligated in the having of a succah and a lulav like Succos. Place - not all places are obligated in the separation of Terumah and Ma'aser and the forbidding of tevel like Eretz Yisrael, or obligated in the bringing of offerings like the Beis Hamikdash. Items - not just any plant can be used in place of the lulav and esrog, and not just any animal can be offered as are cows, sheep and doves, and not everyone is fitting to perform the sacrifices as is a Kohen.
I cannot explain more than this, but an intelligent person will understand my words.
What the Rashba means is that Yaakov was able to ascertain with his great wisdom that according to the time and the place and the items (the two sisters), he was not forbidden to marry them.
Ramban to Bereishis 26:5 says the Avos only kept the Torah in the land of Israel (see @user688's answer)
Parshas Derachim and Maharsha to Yoma 28b say as converts to Judaism Leah and Rachel were no longer siblings (see @AdamMosheh's answer)
Gur Aryeh to Bereishis 46:10 says a similar idea, that the observance of the mitzvos of the Avos had the "geder" of "tinok shenolad dami", as if they were converts. He later says that they only kept the positive mitzvos, not the negative ones.
Teshuvos HaRashba I § 94 with the explanation of Teshuvos Radvaz § 696 gives a more kabbalistic explanation, seemingly what Nefesh HaChaim in Sha'ar Alef Chapter 21 means (see @user4523 and @Menachem's answer)
Da’as Zekeinim to Bereishis 37:35 say that the Avos using their prophetic powers were selective about which mitzvos to observe. Rav Asher Weiss wants to say they meant what the Nefesh HaChaim meant.
Ohr HaChaim to Bereishis 49:3 says a prophet can at times of need nullify a mitzvah
Rabbi Eli Mansour explains that there is a special kabbalistic tikkun that had to be made by Yaakov marrying the two sisters.