Among the many explanations of what it means when we say that the Avos kept the Torah (as in the linked question) is that they did so on a spiritual level. R. Shneur Zalman of Liadi thus states in one of his Chassidic discourses (Va'eira 5572):
קיים אאע״ה כל התורה כולה עד שלא ניתנה וכן ביעקב עם לבן גרתי ותרי״ג מצות שמרתי והיינו רק ברוחנית שהרי לא יתכן שקימו מצות חליצה ויבום ופרה אדומה כו׳
"Our forefather Avraham kept the entire Torah before it was given, and so too with Yaakov, 'I sojourned with Lavan and kept the 613 mitzvos'. This is only on a spiritual level, since it is impossible for them to have carried out the mitzvos of chalitzah, yibbum, parah adumah, etc."
- and he goes on to explain that in this sense they reached only a "lower" level of closeness to Hashem, unlike us, their descendants, who are able to perform these mitzvos physically and thereby connect with Hashem's Essence.
So according to this approach, the fact that Yaakov in fact married two sisters is neither here nor there: he accomplished that mitzvah in a spiritual dimension, just like all of the others.
How so? The Tzemach Tzedek (in Derech Mitzvosecha, Issur Ervas Achos Ishto 30a-b) explains that Yaakov's marriage to Rachel and Leah - and indeed, all of the forbidden relations in the Torah in general - in fact are reflective of the "unification" of enormously great Divine energies in the world of Atzilus, energies which are too intense to descend into the lower worlds - including our own - without being perverted into evil. He continues:
אלא שיעקב לפי שהי' לפני מ"ת והי' טהור וקדוש עשה כן דוגמת היחוד באצי' כי לו יאתה לעשות כן להיותו מרכבה למדות דאצי' וגם לאה ורחל הם מרכבה כנ"ל ועדיין לא ניתנה תורה בבי"ע לאסור לעשות כן למטה
"But Yaakov - living as he did before the Giving of the Torah, and being pure and holy - did this following the model of the unification in Atzilus. It was appropriate for him to do so, since he was a 'chariot' for the attributes of Atzilus, and Leah and Rachel too were a similar 'chariot,' as explained earlier, and the Torah had not yet been given in the [lower worlds of] Beriah-Yetzirah-Asiyah to forbid this to be done below."
Meaning, then, that Yaakov lived and observed Torah as it is in Atzilus; what actually happened here on earth could not affect that higher spiritual world. That began only with the Giving of the Torah, when Hashem declared that "the lower ones can ascend above, and the higher ones can descend below" (Shemos Rabbah 12:3).