Mario Gramsci (1893-1945) was the only Fascist member of his family. Antonio had broken off relations with him in 1921 and things apparently remained that way, except for a brief reconciliation in 1927. (See Antonio Gramsci's letter to his mother of 23 May 1927, and also Mario's letter to Antonio of May 17. The letter is in Nuove lettere di Antonio Gramsci, edited by Antonio A. Santucci [Rome: Ed. Riuniti, 1986], pp. 82-83.)
According to Giuseppe Fiori (Eng. ed., p. 301, n. 5), Mario "enlisted in the army during the Abyssinian war, and later fought in North Africa. He was captured and spent several years in an Australian prisoner-of-war camp. He died in 1945, at the age of 52, immediately after his return from Australia from a serious illness contracted during imprisonment. He left two children, Gianfranco and Cesarina."
The only monographic piece on Mario Gramsci and his family which I recall is #5468 of the Bibliografia gramsciana: Giovanni Ruggieri, "Antonio Gramsci, nostro zio (Un eccezionale incontro con i nipoti del rivoluzionario sardo)", Gente, 19 (1975), pp. 8-10. A most recent publication has given us further information on this question. It is an interview with Cesarina Gramsci, Antonio's niece and Mario's daughter: Cesarina Gramsci, «Mio padre, il Gramsci nero [Interview by Roberto Di Caro]», L'Espresso, xliii, 19 (15 May 1997), 84-85. She seems to be a decent person, but has a tendency to avoid problems and minimize differences. For her, Uncle Antonio was not a hunchback and her father was not really a Fascist.
Though the comparison of Antonio's and Mario's lives--as it has been made by some in the name of "national reconciliation"--is obscene and misleading. There is no denying the tragic character of the younger brother's life. It is grotesquely appropriate to the more brutal aspects of our century. He was a volunteer in World War I, a volunteer in the Ethiopian war, and again in World War II (at the age of 47!). And in between these disasters he was an enthusiastic volunteer to the very ideology which did him in! What a life!