At the time they had stringent codes for acceptance in the Cosa Nostra. You had to be fully Sicilian (both parents). Then when Charles "Lucky" Luciano took part in the Commission he said "we have to stop this Sicilian thing. We're killing eachother over which part of the country we're from. This crap is giving us a bad name". From then on, you only needed to be fully Italian, but not from Sicily (as we see, one of the more famous mafiosi, John Gotti, was Napolitean. Nowadays, you only need to have an Italian father to be part of the Cosa Nostra. Shows that it's really gone downhill huh?