"I began to become disenchanted when I saw so much injustice," Juanita Castro said in an interview with Maria Antonieta Collins, the co-writer of her memoir.
Juanita Castro, who has not had contact with her brothers for more than four decades, said that before her exile to Miami she had worked from her home in Havana to shelter and help those who she said were being persecuted by Fidel Castro's government.
|Fidel Castro stood down as Cuban leader in 2008 after 49 years in power [Reuters]
"My situation in Cuba became delicate because of my activity against the regime," she said.
She received an invitation from the CIA shortly after the US's failed Bay of Pigs invasion in 1961, asking her to collaborate by collecting information on the Castro leadership, she told Collins.
"They [the CIA] wanted to talk to me because they had interesting things to tell me, and interesting things to ask me, such as if I was willing to take the risk, if I was ready to listen to them - I was rather shocked, but anyway I said yes," Juanita Castro told Collins.
Collins said that "in this way began a long relationship with the arch-enemy of Fidel Castro, the Central Intelligence Agency."
"During three years, from 1961 to 1964, at the risk of her own life, the work of Juanita Castro was to save the lives of her compatriots long before she left for exile in Miami," Collins said.
Juanita Castro, who ran a pharmacy before retiring in 2006, last spoke to Fidel at her home in Havana in 1963 when their mother, Lina Ruz Gonzalez, died of a heart attack.
She last spoke to Raul in 1964, days before she went into exile, she said.
Fidel Castro, 83, established a one-party communist system in Cuba after the 1959 revolution and ruled the island for nearly half a century.
He handed over the reins to Raul, 78, last year following his ill-health.