Now, as I've posted elsewhere several times, the novel states that Sonny and Tom were like actual brothers, but that Michael and Tom were not close at all.
Indeed, when Tom is released by Solozzo and returns to the compound, Sonny is so happy and relieved that Michael realises that he can never be as close to Sonny as Tom is. He is jealous and resentful at this realisation.
One thing neither the novel nor the film explore is the relationship between Tom and Fredo. The only real interaction I can remember is from II, when Fredo asks if he can talk to Mike and Tom tells him rather coldly "No Chance."
