Well, as it turns you, you don't even need a family insider, just Wiki:

In 1931, both 45 year-old Frank Nitti and 32 year-old Al Capone were convicted of tax evasion and sent to prison; however, Nitti received an 18-month sentence, while Capone was sent away for 11 years. Nitti was not a troublesome prisoner, but found the year-and-a-half confinement in a cell difficult because of the closed-in space. When Nitti was released in 1932, the media hailed him as the new boss of the Capone Gang.[5]
In truth, however, Nitti was only a front man. According to crime historians, "it was ludicrous" to expect that people such as Tony Accardo, Jake Guzik, Murray "The Camel" Humphreys, or Paul "The Waiter" Ricca would have taken orders from Nitti. By all accounts, Ricca had the real power by at least 1932 and was clearly the de facto boss by 1939 even though he was technically Nitti's underboss. When Charles "Lucky" Luciano and Meyer Lansky organized the National Crime Syndicate in the 1930s, they considered Nitti a human cypher. Lansky and Luciano dealt with Ricca, not Nitti, as the boss of the Chicago Outfit.[5]