I don't believe that Carlo was anything above an associate at best. Vito even told Tom to "give him a living" and not to discuss Family business with him. Vito distrusted him because he wasn't full blooded Sicilian and he only consented to his marriage to Connie on the condition that they have a traditional Sicilian wedding. Nobody in the Family had any respect for him,and certainly could care less about Sonny beating his brains out. He ran a small gambling business for the Corleones and he even screwed that up.