 His parents were Giuseppe Emanuele "Joe" Rosselli (1867 - 1911) and Amelia Pincherle (1870 - 1954) , who was the paternal aunt to writer Alberto Moravia; Nello was the last of three sons, the others being Aldo Sabatino (1895 - 1916), died in WWI and Carlo Alberto (1899 - 1937).Nello was a member of the reformist Unitary Socialist Party of Filippo Turati, Giacomo Matteotti and Claudio Treves, which had split from the PSI.After the rise of Fascism, he fled to France with his brother, and from there was active in anti-Fascist and socialist politics, helping to found the group Giustizia e Libertà and aiding the Republicans in the Spanish Civil War, as well as carrying out propaganda missions within Italy.