Rashi to 45:4 says that when Yosef told his brothers to approach him, it was to show them his circumcision (presumably to prove he was Jewish). But how would that prove anything? All of Egypt had been circumcised already, as Rashi says on 41:55. If his showing his circumcision wasn't to prove he was Jewish, why did he do it?
Chizkuni asks this and offers two answers:
The Tzeida Laderech adds two more explanations: