Interesting question, Mick. If you're referring to American Mafia families:
Prior to Prohibition, organized crime in America was small-time and local. Every city had a "red light district" where "vice" (prostitution, gambling, drugs, illicit booze since many communities were officially "dry" before Prohibition) flourished. In ethnic neighborhoods (especially Chinese, Irish, Jewish and Italian), crimes of extortion flourished. In Italian neighborhoods, the racketeers were most often Sicilian or Neapolitan, since that's where most Italian immigrants to the US came from. The Sicilians were always better organized because they were more clannish than the Neapolitians. But they were essentially small-time. They didn't really call themselves "Mafia" officially. Rather, they saw themselves as "men of honor," or "men of a tradition," as they thought of themselves in Sicily. Omerta was part of their tradition.
Jewish gangs (and to a lesser extent, Irish gangs) dominated the Prohibition rackets in nearly all American cities except Chicago. There, the Torrio-Capone "Outfit" was the biggest (but not the only) gang. Even though most members of the Capone gang were Italian, he never considered it a "Mafia family," and many non-Italians held high places in his mob. As I mentioned many times before: there was a Mafia in Chicago called the "Unione Siciliano" that was like a civic association with muscle. Capone respected the Unione, but the two never competed.
To get to the point (and I'm certain you wish I would ): Following the Castellemmarese War of 1930-31, Charlie Luciano coalesced the US Italian mobs around a clearly defined set of families and a family structure that had been established just a few months earlier by Salvatore Maranzano, who was assassinated on Luciano's orders. Luciano didn't publish a set of "rules" or "behaviors" for Mafia families. They didn't send away to Sicily for charter certificates in "the Mafia." Nor did they even call themselves "Mafia" officially (newspapers did that). Rather, Luciano and his colleagues agreed that competition was bad for the organized crime business, so they worked to establish monopolies on rackets in cities or neighborhoods. They also decided that it would be best to restict membership to Italians (most were all-Italian anyway), because they were most comfortable with each other; although, following Luciano's example, they drew liberally on non-Italians for advice and help.