When Yosef revealed himself to his brothers, they could not respond, as they were in a state of shock and disbelief. Rashi tells us that he proved he was their brother because he indicated that he was circumcised.
Rav Kook felt that there was a much bigger message that Yosef was teaching his brothers. Despite living in the most decadent place on earth, he still managed to observe all of the Mitzvot. ( The Rambam wrote that Egypt was so decadent that men married........