The case stems from a wider inheritance dispute between the Elkanns and their mother Margherita over the estate of Gianni Agnelli, which has divided one of Italy’s best known business dynasties.
Prosecutors in the northern city of Turin said in a statement that their investigation showed that Caracciolo was a resident in Italy from at least 2010 and not Switzerland, and that her inheritance therefore should have been taxed in Italy.
They said claims she had been based in Switzerland were part of "a criminal plan to hide her substantial assets and related income from Italian inheritance and tax laws".
Lawyers of the Elkanns said the seizure was a procedural step in the case, which does not signal any liability by the defendants.
"We reiterate that Marella Caracciolo had been resident in Switzerland since the early 1970s, well before the Elkann brothers were born. Her will to reside in Switzerland has never failed throughout her life," they said in a statement.
Lawyers said they "remain convinced that we can prove that our defendants have nothing to do with the facts charged".
Other people targeted by the seizure are John Elkann’s accountant Gianluca Ferrero and Swiss notary Urs von Grunigen, who acted as estate executor.
John Elkann is also the CEO of the Agnelli family holding company