That's how Michael introduced Kay to Tom at Connie's wedding. But, as GF and II progressed, I think it was pretty evident that Michael didn't really regard Tom as his brother. Even right after the Tahoe shooting, when Michael was putting Tom in charge, and he said, "Tom, you're my brother," Tom practically broke down in tears when he replied, "I always wanted to thought of as a brother by you, Mikey -- a real brother." Tom's statement tells me that he knew Michael never regarded him as "a real brother." We see a very explicit example in the flashback scene at the end of II, after Michael announces that he's joining the Marines. Tom says that he and "your father [not "Pop" or "our father"] have discussed your future many times." Michael replies, "You talked to my father about my future?" Not exactly brotherly talk...

Michael grew up with Tom in the same household, with Tom being treated by his father as a full member of the family--and with Sonny (and probably Fredo) accepting him as a real brother. Why didn't Michael accept Tom as a real brother?