When Yosef went to find his brothers by his father's order and did not find them where they were supposed to be, a man (who was actually the angel Gabriel) found him and asked what he was seeking. Yosef said he was looking for his brothers, and the verse says the man told Yosef: "They have moved from here; for I heard them say, 'Let us go to Dotan.' So Yosef went after his brothers and found them in Dotan" (Bereshit 37:17). Rashi comments on the phrase "They have moved from here" that the angel was hinting to him: "They have departed from brotherhood."
One can explain the meaning of this expression—"They have departed from brotherhood"—according to the explanation in the book Parashat Derachim (Derush 1). It explains that the foundation of the argument between Yosef and his brothers was whether their status was fully that of the [People of] Israel—even to apply the law flexibly when appropriate—or if they still had to conduct themselves with the severity of the laws of Bnei Noach (descendants of Noah), like the rest of humanity.
Yosef maintained that they were still considered as Bnei Noach and, therefore, had to be strict. In contrast, the Tribes maintained that they were already fully considered the People of Israel, both to be strict and to be lenient.
In the Talmud (Sanhedrin 58b), it is taught that non-Jewish nations (Bnei Noach) do not have legal kinship through the father, but only through the mother. This explains the words of the "man": that at that moment, his brothers had decided to judge Yosef according to Yosef’s own opinion — that is, to treat him completely as a Ben Noach. As a consequence, "they departed from brotherhood" with him, because, according to that logic, they were only relatives through the father, and among Bnei Noach, paternal kinship is not considered kinship. And since there is no kinship, they could proceed to kill him.
That is why, when Yehudah came to rebuke them to prevent them from acting in such a cruel manner, he based his words on the fact that Yosef was their brother. For the entire reason they wanted to kill him and had departed from brotherhood with him was that they had judged him as a Ben Noach according to Yosef's own stance. Therefore, Yehudah told them: "What do we gain by killing our brother...? Let not our hand be upon him, for he is our brother, our own flesh."
This was not merely an emotional expression, but a direct response to their argument: "Even if you want to treat him as a Ben Noach according to Yosef’s own position, nonetheless, we must treat him according to our opinion, as a Ben Israel, meaning he is truly our brother, and we cannot shed his blood with our own hands."
Zera Shimshon, Parashat Vayeshev, Art. 10
