I brought up this subject when I first joined the Board years ago. The answer I got was that Puzo used the term Five families as a general reference and did not mean that there wer five families in addition to the Corleones. I accepted that the Corleones were one of the Five Families. Yes, in the novel there are plenty of references to the Five Families by the Corleones. But, again, I accept that Puzo used that term a s general reference to include the Corleones.