She was involved in gathering secret information for the US to overthrow the Cuban government.
Juanita Castro, the younger sister of former Cuban leaders Fidel and Raul Castro, has died in Miami at the age of 90. Juanita Castro had lived in the US since the mid-60s, where she fought to overthrow her brothers and change the Cuban regime, the BBC reports.
"Juanita Castro, an extraordinary woman and tireless fighter for the cause of the Cuba she loved so much, died today," Maria Antoñeta Collins, co-author of Castro's autobiography, wrote on Instagram.
Juanita Castro left Cuba in 1964, a year after her mother's death, due to disagreements with her brothers following the 1959 Cuban revolution. She settled in Miami, where she ran a pharmacy until 2007.
Juanita Castro actively supported opponents of the Cuban regime and even collaborated with the CIA.
As she wrote in her memoirs, she became disillusioned with her brothers for what she saw as their betrayal of democratic principles and the assassination of their political opponents.
After the revolution, she helped and hid Cuban dissidents until 1964, when she left the island herself.
In her memoirs, entitled "Fidel and Raul, My Brothers, The Secret History", Castro claims she was recruited by the CIA in Havana two years after the 1959 revolution that brought Fidel to power. Code-named Donna, she spent three years gathering secret information for the United States in an attempt to overthrow the Cuban government.
Juanita Castro wrote in her memoirs that it was not easy for her to find her place in the expatriate community in the US and to combine her two roles - sister of Fidel and Raul and opponent of the regime in exile.
"It was undoubtedly more difficult for me than for many others who left. Few people understand the paradox of my life," she wrote.
"To those in Cuba, I am a deserter because I left and denounced the current regime. And to many in Miami, I am 'persona non grata' because I am the sister of Fidel and Raul.
Her older brother Fidel died in Havana in 2016 at the age of 90. Juanita did not attend the funeral.
Raul Castro is now 92 and continues to live in Cuba.