Why did Joseph say to his brothers, "I am Joseph, and this is my brother," even though they were all his brothers?

The meaning of Yusuf's statement, "And this is my brother," refers to his full brother Benjamin, whereas the others were brothers only from the father's side. Ibn Kathir mentioned that Yusuf introduced himself to his brother Benjamin and told him not to grieve over what the brothers had done. He then instructed him to keep the matter secret from them and conspired with him on a stratagem to keep him with him, honored and dignified.