What did Yosef do that surprised the brothers?
Rashi and Targum Yonasan: He struck his goblet and called by name the sons of Leah, in order of age, to sit together; and likewise for the sons of Bilhah and the sons of Zilpah. He concluding by saying that, since neither Binyamin nor himself had a mother, they too would sit together. 1
Da'as Zekenim, Hadar Zekenim, Moshav Zekenim, Rosh: They were baffled why Yosef sat, neither with them nor with the Egyptians, causing them to wonder as to his identity. 2
Rashbam, Moshav Zekenim: They wondered how he knew their order of birth, seeing as (except for Binyamin) they had all been born within seven years (really, six - PF) of each other, and it would have been all but impossible to work it out on one's own (all were about 40-45 years old).
Why does it say, "ha'Bechor ki'Vechoraso," and not simply "k'Soledosam"?
Ha'amek Davar: There were three Bechoros (to their mother). Yosef knew to seat them properly - Reuven was not a full Bechor, for Yehudah was the leader. Also "ha'Tza'ir ki'Tze'iraso" was not purely based on age - Binyamin was youngest, but he was esteemed.
QUESTIONS ON RASHI
Rashi writes: "He tapped his cup and announced... 'Reuven, Shimon....' When he reached Binyamin, he said, 'This one has no mother....'" But Leah must also have passed away already, for she was buried in Me'aras ha'Machpelah (see 49:31), presumably before Yaakov and his family descended to Egypt? If so, most of the brothers present had no mother!
Gur Aryeh: Yosef meant that (like himself,) Binyamin had no mother even during his childhood, growing up - when a son needs his mother.
