I don't think Fredo looked up to Sonny. In the novel it is stated that Sonny was a rebellious and wild teen while Fredo was well-behaved. He seems to side with Michael, most notably when Michael joins the Marines. I don't recall any scene in the film or novel in which Sonny and Fredo spoke or behaved like brothers as did Fredo and Michael. Of course, I may be wrong. I haven't seen the film or read the novel as many times as I should have.