Sure, there were plenty of sibling rivalries--there always are in families. The rivalries are one of the factors that make the Trilogy such a richly satisfying family saga. But what really complicated matters in the Trilogy is the fact that Tom is also, in effect, an employee of the family as well as a brother. IMO, the most revealing example is in the flashback scene at the end of GFII, when Michael announces that he's joined the Marines. Sonny reacts with violent anger, much in character for him. Tom also reacts in character--as the Don's consigliere-in-training. "Michael, your father and I have discussed your future many times..." Michael gives him that patented Michael Corleone cold stare: "You...discussed my future...with my father..." Of course he did--that was part of Tom's role as consigliere-in-training. But it made for supreme awkwardness because Tom was also technically Michael's brother.