Surely the brothers did not need to give their number, 12, nor the supposed fate of Joseph and the fact that Benjamin is with their father, who is alive. It is hard to understand indeed why they thought it relevant to mention all this.
This is indeed so striking that Jacob demands, in the next chapter, “Wherefore dealt ye so ill with me, as to tell the man whether ye had yet a brother?” (Gen 43:6) But in answer to their father, the brothers actually provide information on this specific narrative detail: “And they said, The man asked us straitly of our state, and of our kindred, saying, Is your father yet alive? have ye another brother? and we told him according to the tenor of these words: could we certainly know that he would say, Bring your brother down?” (43:7)
Now, it seems possible that they are now lying to Jacob, because we see no mention of this further questioning in all of 42:7-25; in particular, when they share personal information at 42:11 and 13, it does not appear to be in response to a question such as, “Is your father yet alive?” and “Have ye another brother?”
But it is very possible that they are telling the truth, because the Bible text is frequently telegraphic, and we have another example in which a more uncontroversial statement is supplied later, at 42:34, which is not reported anywhere in 42:7-25. In other words, the additional detail at 42:34 is solid evidence that 42:7-25 is not a complete account of every bit of conversation that passed between Joseph and his brothers; indeed, the remark that Joseph “communed [conversed] with them” (42:24) alone establishes this point, because the contents of that conversation are not revealed.
Besides, there is no other particularly good reason why the brothers would simply volunteer the information without prompting.
Still, the story of the brothers’ meeting with Joseph was originally told (42:7-25) and then retold more briefly to Jacob (42:29-34), but in neither version were the prompting questions by Joseph included. Why not?
As I have said, the Bible text is frequently telegraphic, reporting details only as necessary and repeating only those that are truly important. The detail of Joseph’s questions (especially “Do you have another brother?”) is admittedly important, but the Bible is hardly short of sensivity to the requirements of narrative drama. By omitting the questions in the first telling, we readers are made to feel Jacob’s consternation as well as his understandable lack of trust in his sons. Indeed, we have even less reason to trust them, because we know about their enormous lie about what happened to Joseph; Jacob still does not.
So it would indeed seem possible, even probable, that they had deliberately betrayed Benjamin by simply mentioning his existence—perhaps out of envy, as they had for Joseph?—if they were not depicted, ten verses later, as saying, “And they said one to another, We are verily guilty concerning our brother, in that we saw the anguish of his soul, when he besought us, and we would not hear; therefore is this distress come upon us. And Reuben answered them, saying, Spake I not unto you, saying, Do not sin against the child; and ye would not hear? therefore, behold, also his blood is required.” (42:21-22) These sincere expressions of contrition surely make it essentially impossible that they deliberately betrayed Benjamin.
Besides all that, we are also told that Judah reminded the vizier of their earlier conversation two chapters later: “My lord [the vizier] asked his servants [the brothers], saying, Have ye a father, or a brother?” (44:19) The author of Genesis is making it abundantly clear that it really was in response to Joseph’s question that they told him about Benjamin. This establishes definitively that they told Joseph about Benjamin only in response to a question. Besides, the brothers were wily and mature enough not to simply volunteer unnecessary personal information without prompting. Probably their worst error was to fail to have the foresight to lie about Benjamin in response to Joseph’s question. But even this failure to lie speaks well of the state of their souls, after 22 years.