Why did Yosef say, "Im Kenim Atem"?
Ohr ha'Chayim: If Nochrim say, give to us one of you (to kill him); if not, will kill all of you, they may not comply (unless the Nochrim designated the person, for then in any case he will die - Yerushalmi Terumos 8:4). If you are honest, you may hand one of yourselves over; there is no danger. If you are lying (and do not bring your younger brother), I will kill the one you leave here; you may not hand him over! Even though 42:24 implies that Yosef picked Shimon, perhaps really, they designated him. Or, after they agreed to give one, Yosef picked Shimon, to separate him from Levi.
Malbim: If you are truly brothers, one of you would be willing to be security until the others return. If not, no one would agree, lest the others not return and he will be Chayav Misah! He would tell the truth (reveal their mission or other information, to try to save himself)!
Ha'amek Davar: Here, it cannot refer to testing their honesty. After this, he says that one will be incarcerated, and the rest will bring food for their families, and then bring Binyamin. Right now, they were all incarcerated! Here, it surely means that they have families [and need to feed them; refer to 42:11:4:3]. Below (42:33), the brothers did not tell Yaakov that they were all incarcerated; one could 1 explain "Kenim" to mean honest; Yosef said to send nine to bring Binyamin to verify their claim, and by the way, bring food to their families.
Ha'amek Davar himself explains also there that "Kenim" means having families.
QUESTIONS ON RASHI
Rashi writes: "'In your prison' - [i.e. the one] in which you are now imprisoned." What is bothering Rashi?
Gur Aryeh: The prison did not belong to the brothers; it belonged to Pharaoh! Rather, this means the one in which they were presently found.
Rashi writes: "'And [the rest of] you, go bring' - ... to your father's house." Is this not obvious?
Gur Aryeh: The command "go bring" usually means to bring something back to the speaker; whereas here, they would be transporting food away from Egypt. Rather, it means "to bring home."
