As Danito indicates, it should be obvious even to the casual viewer of the film that what FFC was doing in that scene was identifying the characters for the audience probably on the assumption that many of them had not read the novel. "my brother" introductions are the primary illustrations of that. In the same vein, identifying Tom Hagen for Kay was also for the audience. It provided a segue into the story of how Tom became part of the family while also illustrating the degree of Michael's discomfort with Tom.