As Chopper implied: The Chicago Outfit was not a Cosa Nostra family, at least not initially. The founder of the Outfit, Johnny (the Fox) Torrio, was a Neapolitan, at a time (ca. 1920) all the Mafiosi in America were Sicilians. There was a Mafia organization in Chicago at the time of Torrio and Capone: the Unione Siciliana, which was a kind of civic association with muscle. Capone respected them, and attempted to put his own, Sicilian-born men into the Unione presidency. But as Chopper pointed out, Torrio and Capone reached out to non-Sicilians and even non-Italians. Jake Guzik ran the Outfit, with Frank (the Enforcer) Nitti, for several years while Capone was in prison.

Tony, a point you might want to consider:

Torrio/Capone's outreach to non-Sicilians and non-Italians was a harbinger of things to come. When Charlie Luciano formed the Commission in 1931, following the Castellemmarese War, he permitted non-Sicilians, like Frank Costello and Vito Genovese, and non-Italians, like Meyer Lansky, Bugsy Siegel and Dutch Shultz, to participate in Commission meetings. Lansky remained the Commission's number-one counsel for decades.