Carlo and Nello Rosselli didn’t look like heroes, or like martyrs. They were portly young men with round faces and sweet smiles. They loved their mother very much: “There is no one on earth equal to our adorable mother,” wrote Carlo to Nello. They also loved their wives. Nello’s career as an academic and biographer won him respect but was much interrupted. Carlo taught for a while too, then lost his job and lived thereafter on income from his family’s shares in a mining company. Yet when they were killed in France in 1937, almost certainly on Mussolini’s orders, the brothers’ funeral cortege was followed to the Père Lachaise cemetery by 200,000 people. Fourteen years later, they were reburied near Florence, and the city’s streets were crammed with admirers. Their children were joined at the grave side by Ferruccio Parri, a former prime minister. Many people had once fervently hoped that Carlo himself would one day hold that post.

If British readers know about the Rosselli brothers at all, it is likely they have glimpsed them through the distorting lens of their cousin Alberto Moravia’s fictional imagination. Moravia’s The Conformist (and Bernardo Bertolucci’s louche and beautiful 1970 film version) tells of the assassination, by Mussolini’s hitmen, of an anti-fascist intellectual living in exile. Like the victim in the film, Carlo and Nello were driving through a wood in rural France when they were ambushed. There was a car, with people standing beside it. Nello, generous man that he was, stopped and got out to see if they needed help. He was shot and stabbed; when Carlo tried to come to his rescue, he was killed too.

The brothers had been brave and outspoken critics of the fascist regime. They published an underground newspaper called Non Mollare (do not give up). They organised defiant stunts and demonstrations. Their homes were broken into by fascist squadristi. Both endured periods of imprisonment. They gave most of their personal fortune to the anti-fascist cause. On trial for “rebellious behaviour”, Carlo delivered a speech that became a kind of litany for anti-fascists everywhere. “I had a house: they destroyed it. I had a magazine: they suppressed it. I had ideas, dignity, an ideal: for these I was sent to prison. I had friends: they killed them.”

Much has been written about life under fascist rule. Richard Bosworth’s several books, in particular, add up to a comprehensive and illuminating account of Italians’ experience of Mussolini’s reign. But the study of a nation can never be as emotionally compelling as the study of a family. To read a historian’s record of the thousands of dissidents sentenced to confino – banishment to remote prison-islands – is illuminating, but to read in detail of the day-to-day experience of one or two of those exiles is far more moving. Expertly alternating vivid domestic detail with lucid exposition of the gradual evolution of totalitarianism, Caroline Moorehead allows her readers not only to know, but also to feel, how it was to endure fascist oppression.

Facebook Twitter Pinterest Carlo Rosselli. Photograph: Hulton Archive/Getty Images

She had two wonderful troves of source material. When separated from their adored “Mammalina”, Carlo and Nello wrote almost daily to her, sharing with her their news and their thoughts. Their father, a compulsive gambler, had faded from their lives when they were still small. Their elder brother was killed fighting in 1916. Amelia’s two remaining boys were her all-in-all. She accompanied them into exile. She sheltered their wives and children when they could not. She taught them that they must accept “responsibility’” (the word recurs in their correspondence) and strive for “serenity”. As Moorehead points out, there is something at once comical and splendid about the way mother and sons assure each other how “serene” they are, even in the most terrible times. Fascist thugs may have broken into the family house, terrifying the servants, smashing precious porcelain and hurling the broken remnants of a piano out into the street. The brothers may have been beaten up or have been facing long prison sentences or mourning the murder of friends. Always, however implausibly, they report: “I am serene.”

This archive of letters, shot through with love and shared political passions, is matched in volume by another huge stash of documents inspired by suspicion and hostility. By the end of his life, Moorehead writes, Carlo had been watched by no fewer than 42 of Mussolini’s secret agents. The letters allow Moorehead to give us a wonderfully intimate and engaging account of the brother’s domestic lives, their marriages, their intellectual development and their enthusiasms. The police files, complementarily, provide her with obsessively detailed accounts of their activities, of who they saw, where they went, which meetings they attended and – most poignantly – how many undercover fascist agents Carlo welcomed into their circle, trustingly confiding in sneaks, agents provocateurs and spies.

In 1926 Carlo helped the Socialist leader, Filippo Turati, object of ceaseless harassment by the regime, to get away by boat to Corsica and thence to France. For his part in the escape, Carlo was sentenced to imprisonment, and then to five years confino on the tiny, parched Sicilian island of Lipari – now an idyllic holiday destination, but then a prison where TB was endemic and malnourished political prisoners sweltered or froze, depending on the season, and fumed with frustration all year round. “Not even the flies think of escaping,” boasted the island’s governor, but Carlo managed to do so. Moorehead tells of letters in invisible ink, of a boat transported across France only to founder when launched in Marseille, of months of anxious waiting and then of a second, successful attempt, at the end of which Carlo and two other escapees chugged at last in their little motor-boat along the coast of Tunisia, while mounted spahis galloped, cheering, along the shore. An exhilarating adventure, but one whose ending could only be partially happy. Carlo could never return to Italy, and his life among the squabbling factions of the anti-fascist Italians in exile was full of disappointments.

Moorehead quotes numerous accounts of how tirelessly he worked, how energetic he was, how eloquent and how much loved, yet it’s hard to assess whether any of his activity had any effect. Nello’s case is simpler. After a spell of confino, he was allowed to rejoin the University of Florence, under promise of good behaviour. He devoted himself to writing about the heroes of the Risorgimento, Italy’s 19th-century nationalist liberation movement, implicitly comparing their noble opposition to tyranny with the cravenness with which most modern Italians were submitting to it. It was his terrible bad luck that Carlo’s killers struck on one of the few occasions that the brothers had been together in five years.

One of Moorehead’s earlier books, about conscientious objectors, was called Troublesome People: the phrase recurs in this one. A prolific author who has always combined seriousness of purpose with a warm, human touch, she is drawn to good-hearted troublemakers. Moorehead has written about the courageous Sicilian magistrates who dared to challenge the Mafia, and about the Red Cross. In two successful recent books, she covered stories about the French Resistance. Italy, though, is her second country (she was educated in Rome). A Bold and Dangerous Family is animated by the evident admiration and affection she feels for her subjects. It feels like the book she was born to write.

Lucy Hughes-Hallett is the author of The Pike, about Gabriele D’Annunzio. Her recently published novel is Peculiar Ground

• A Bold and Dangerous Family: The Rossellis and the Fight Against Mussolini is published by Chatto and Windus. To order a copy for £15 (RRP £20) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.