Read Genesis 44
This seems like a complicated plan but I think that in the end Joseph wanted to see how his brothers felt about their father. Benjamin was certainly a most favored son and should he die it would break Israel's heart. It was obvious from previous chapters that there was little love from Israel to his sons but Joseph is testing if there is love form his brothers to his father. And what a change in Judah, it was he that had the idea of selling Joseph to the travelers.