You have misunderstood Don Giorgio Gambino. I'm pretty confident he's referring to the novel The Family too. There's some incestuous goings on in the book and he uses the example of Godfather III's Mary and Vincent relationship as a comparison.

Anyway, the book seems to get mixed reviews. Some people can't get into it, others like me find a pretty good read. Although as my 5th grade teacher always taught me, a book should have a beginning, a middle and an end. The Family, to me has no such direction. That aside though, yeah it's good and opened my eyes to the Renaissance period of history.