In Devarim 21:16 - 17 it states that the father cannot favor the son of the beloved wife over the older son of a hated wife.
If we assume the notion that our forefathers performed all the mitzvot of the Torah, how was it possible for Ya'akov to give Yosef, who is the oldest son of his beloved wife, Rachel, a double portion (see Breishit 48:22 and Rashi's explanation, there) instead of giving it to Re'uven who was the true first born of the "hated" wife, Le'ah?
I heard once from Rabbi Dovid Fohrman that this was exactly the question that the sons of Yaakov were challenging him with.
Bereishis 37:32
וישלחו את-כתונת הפסים, ויביאו אל-אביהם, ויאמרו, זאת מצאנו: הכר-נא, הכתונת בנך היא--אם-לא
And they sent the cloak to him, and it was brought to their father, and they said, "We found this. Recognize, please, is this the cloak of your son or not?"
Devarim 21:17:
כי את הבכר בן השנואה יכיר לתת לו פי שנים בכל אשר ימצא לו כי הוא ראשית אנו לו משפט הבכרה
The firstborn, son of the hated wife, he shall recognize, to give him double in everything he finds to himself, as he is the first of his strength, to him goes the firstborn rights.
The cloak was representative of the double portion, as it was an additional gift that Yaakov gave to Yosef. The brothers described it as something "found" which they wanted him to "recognize," which is exactly how the Torah describes the rights of the firstborn. The brothers were hinting to Yaakov that it was not right of him to favor Yosef in this way.
In other words, the sons of Yaakov felt that your question was a valid challenge to Yaakov's behavior.
Indeed, in Devarim 21:16 - 17 it states that the father cannot favor the son of the beloved wife over the older son of a hated wife. But that is with all else being equal, with the only reason one favors the younger firstborn being love for the more beloved wife.
Reuven lost his firstborn rights for sleeping with Bilhah, his father's concubine. This is stated explicitly in Divrei Hayamim I, 5:1-2:
א וּבְנֵי רְאוּבֵן בְּכוֹר-יִשְׂרָאֵל, כִּי הוּא הַבְּכוֹר--וּבְחַלְּלוֹ יְצוּעֵי אָבִיו, נִתְּנָה בְּכֹרָתוֹ לִבְנֵי יוֹסֵף בֶּן-יִשְׂרָאֵל; וְלֹא לְהִתְיַחֵשׂ, לַבְּכֹרָה. born. ב כִּי יְהוּדָה גָּבַר בְּאֶחָיו, וּלְנָגִיד מִמֶּנּוּ; וְהַבְּכֹרָה, לְיוֹסֵף.
1 And the sons of Reuben the first-born of Israel--for he was the first-born; but, forasmuch as he defiled his father's couch, his birthright was given unto the sons of Joseph the son of Israel, yet not so that he was to be reckoned in the genealogy as first-born. 2 For Judah prevailed above his brethren, and of him came he that is the prince; but the birthright was Joseph's.
As I note in this post, this idea roughly corresponds to the legal codes in the time of Yaakov. We see in the Code of Hammurabi:
- If a free man has sexual relations with his father's first wife, who is the mother of sons, after the death of his father, that man shall lose his paternal inheritance.
- If any one be surprised after his father with his chief wife, who has borne children, he shall be driven out of his father's house.
In this case, it was a concubine during his father's lifetime, and he did not lose the entirety of the inheritance, but just the firstborn rights. Still, it is of a similar nature.
So, even if we want to say that this Torah law applies to Yaakov prior to the giving of the Torah, there is a cultural and legal context in which to apply the law, just as is stated in Divrei Hayamim (and quite possibly in the ambiguous and cryptic parallel text in parshat Vaychi).
