I think I can shed light on this family tall tale, and it might be at least partially based in truth.

Article 68 of the Italian Constitution of 1946 qualifies Parliamentary Immunity: Members of Parliament are protected search, seizure, and arrest unless they are caught in the act of flagrantly committing a crime (e.g. an MP who has had far too many glasses of wine at a dinner with constituents can still find himself in the drunk tank at the end of the night). This isn’t a blanket immunity either: Parliament can always vote to authorize judiciary authorities to proceed with investigations and arrests (although the mere requirement of this step can be argued to discourage investigations from proceeding in all but the highest-profile cases).

The only thing from which Members of Parliament are expressly safeguarded from persecution is anything that has to do with how they vote - an MP’s voting record is expressly forbidden from being used as evidence or accusations.

Parliamentary Immunity isn’t a uniquely Italian concept. It is a product of the 17th century Enlightenment, when principles like freedom of speech began getting codified in western legal canon, and emergent representative democracies sought to safeguard the autonomy of their elected officials. The English Bill of Rights seems to have been the first to have codified the principle of Parliamentary Immunity in 1689 (following the Glorious Revolution) with the goal of guaranteeing Members of Parliament’s ability to express their view without fear of reprisal from the monarch.

The French constitution of 1791 included a similar provision protecting members of the National Assembly from arrest for anything done in the exercise of their office (but also affirming that criminal accusations did not fall under this prerogative).

In Italy, when the Piedmontese constitution was proclaimed in 1848 the texts’s 67th article proclaimed immunity for members of the newly-instituted parliament. This 1848 text, and the immunity enshrined in article 67, would be extended to all of the peninsula when the Italian Kingdom was proclaimed in 1861.

This did create interesting legal situations given Italy had undergone over a decade of revolt and war over the course of which different factions of activists had worked towards different political goals, especially as the process of unity had been significantly helped along by the process where more moderate or established politicians co-opted or redirected the energies of more radical political currents. While the country was ultimately unified under the Savoyard (Piedmontese) monarchy, many activists all over the peninsula (even in the Kingdom of Piedmont itself) had in fact fought for the institution of a Republic, and while most activists did “settle” on the country’s unification as a Constitutional Monarchy, there continued to be strong “Republican” factions represented in Parliament who believed in limiting the power of the monarch. From the start, these factions contained members who had gotten in trouble for their anti-monarchial activities: the very first Piedmontese legislature in 1849 featured the prominent activist Didaco Pellegrini, who when elected was incarcerated for having been involved in violent revolts in Genoa. After the unification of Italy, as early as 1862 elected members of parliament Antonio Mordini, Nicola Fabrizi e Salvatore Calvino had serious questions asked of their role in episodic confrontations between Garibaldi’s Redshirts and Piedmontese Army regulars, sparking a running debate (should they be investigated for treason or not? As per the aforementioned article 67, Parliament would have to hold a vote) which eventually immobilized Urbano Rattazzi’s government to the point of prompting the Prime Minister’s resignation. But by and large, the new Italian parliament was disinterested in settling scores from the frantic days of war and revolt: most of Garibaldi’s volunteers had been enrolled in the regular army, while many of the officers had been elected to public office; in this way virtually all had taken an oath of loyalty to the monarch.

It took awhile for the country to progress past the phase where the Kingdom’s frantic formation had created a gray area between “Activism” and “Treason.” Bursts of violent activism, with varying degrees of involvement of political figures, would carry on as late as 1869, when parliament voted to allow investigations to proceed against Cristiano Lobbia, Member of Parliament for Thiene-Asiago who was involved in a bizarre case whereby he appeared to have staged his own assassination. While the Hon. Lobbia was absolved in appeal, parliamentary positions taken in committee and on the floor would form the principal precedent for management of parliamentary immunity in Italy.

The late 19th century firmly established the precedent whereby legal authorities were deferential towards Parliament for authorization to begin investigations on MPs (previously, Article 67 was not invoked when Parliament was in not session) and parliament’s self-regulatory powers were reinforced via new legal precedents whereby the ministry of justice and president of the chamber were cut out of the process of presenting votes to waive immunity (individual MPs could initiate votes to allow, or block, investigations).

Halfway through the 1880s, a new question was breached: What if a fugitive or incarcerated person were to be elected? Prior to then, situations (like that of 1849) had only ever involved people awaiting trial, never convicted persons. Some parties, mostly on the political fringe, did put convicted or wanted activists in the electoral lists as statements of intent or solidarity towards prominent figures, with interesting consequences: In 1883 Giovanni Falleroni, an activist who had taken refuge in Switzerland since being wanted for spreading anti-monarchal pamphlets, managed to get elected MP for Macerata. Fallerioni showed up in Rome just to make a show of refusing to take his oath of office, afterwards speeding back to Switzerland before he could be arrested. In 1886, the anarchist-adventurer Amilcare Cipriani was elected in spite of serving a sentence for a murder committed some years before in Egypt, however convicts for serious crimes suffer blanket bans from public office and Cipriani was never sworn-in.

Other politicians did manage to leverage parliamentary immunity to get out of sticky situations, such as for fugitives residing abroad or cases which were awaiting an appeal. In 1882, socialist leader Andrea Costa ended his exile upon his election to parliament, and in 1886 the socialist activist Alcibiade Moneta was also able to end his run from the law thanks to his election to Parliament (he was wanted for inciting a riot). The notable journalist Francesco Coccapieller, known as ‘Sor Checco, was also released from prison upon his re-election in 1886 (he had been elected for the first time in 1882, but during the election season he managed to get himself incarcerated: while dining, he was spotted by a rival politician offended by an editorial, who pulled a pistol on him and in response ‘Sor Checco drew his own weapon and wounded the aggressor) while in 1885 Pietro Sbarbaro, jailed for defamation, was also able to escape prison thanks to his election. Many times, public opinion was actually in favor of parliamentary immunity, especially with regards to figures like Pietro Sbarbaro who led virulent press campaigns against corrupt politicians.

So without going into detail of every single debate which involved a Member of Parliament in legal trouble (every one of which added to the legal precedent and tweaked the process by which immunity was applied), after the Second World War the country drafted a new constitution in which Article 68 continued to enshrine parliamentary immunity, which continued to be seen as an inviolable principle especially given the country had lived through two decades of political repression under the fascist dictatorship.

My main source is Venturini’s, “Cronache di un conflitto tra Parlamento e magistratura nell’Italia liberale: l’interpretazione dell’art. 54 dello statuto sull’immunità dei deputati” (2014).