The first place to start (well, maybe the second place) when discussing hte GF is the novel. It has answers to many questions asked on this Board. Also, it's helpful to read a non-fictionnovel about the Mafia. There's a million out there. By doing so you get a feel for the structure of organized crime.

Tom was Vito's right hand man. That he was raised in Vito's home made him even more so. If you recall from the film, Vito is provided info and advice by Tom about Sollozzo. So, that should be a clue aboput his family status. Sonny asks Tom later what they should do after Vito is shot. That should be a clue also. Tessio and Clemenza are Vito's generals. You never really see Vito or Sonny asking Tessio or Clemenza for advice. If Tom is that close to Vito and Sonny, then it's safe to assume that he can give orders to the generals as the Don's representative.

Again, it's helpful to read the novel.