Juanita Castro, the sister of Cuban rulers Fidel and Raúl Castro who worked with the CIA against their communist government, has died in Miami at 90. Florida had been her home since shortly after fleeing the island nearly 60 years ago.

Journalist María Antonieta Collins, who co-wrote Juanita Castro’s 2009 book, "Fidel and Raúl, My Brothers. The Secret History," wrote on Instagram that she died on Monday.

"Juanita Castro was ahead of us on the path of life and death, exceptional woman, tireless fighter for the cause of her Cuba that I love so much," Collins wrote.

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The Cuban government and media had not mentioned her death as of Wednesday.

In her book, Juanita Castro, a staunch anti-communist, wrote that she began collaborating with the CIA shortly after the United States botched the Bay of Pigs invasion in 1961.

She had originally supported her older brothers' efforts to overthrow dictator Fulgencio Batista, raising money and buying weapons. She became disillusioned when Fidel Castro became a hard-line communist after taking power in 1959 and pushed those who disagreed out of his government.

When her home in Cuba became a sanctuary for anti-communists in the early 1960s, Fidel Castro warned his sister not to get involved with the "gusanos," or worms as the government called those who opposed the revolution.

She said in her book that it was the wife of Brazil's ambassador to Cuba who persuaded her to meet with a CIA officer during a 1961 trip to Mexico City. She said she told the agent that she didn't want any money, and would not support any violence against her brothers or others.

She said the CIA used her to smuggle messages, documents and money back into Cuba hidden inside canned goods. They communicated with her via shortwave radio, playing a waltz and a song from the opera Madame Butterfly as signals that her handlers had a message for her.

She remained on the island while their mother was alive, believing that protected her from Fidel's full wrath.

"My brothers could ignore what I did or appear to ignore it so as not to hurt my mom, but that didn’t mean I didn’t have problems," she wrote. After their mother died in 1963, "everything was becoming more dangerously complicated."

Castro fled Cuba the following year, after Raúl helped her get a visa to Mexico. She never saw her brothers again.

"I cannot longer remain indifferent to what is happening in my country," she told reporters upon her arrival in Mexico. "My brothers Fidel and Raúl have made it an enormous prison surrounded by water. The people are nailed to a cross of torment imposed by international Communism."

Because her work for the CIA had been clandestine and not publicly known, many Cuban exiles feared she was a communist spy when she arrived in the U.S. a year later. She later helped found a CIA-backed nonprofit organization that worked against the Castro government.

She eventually settled into a quiet life in Miami, where she ran a Little Havana pharmacy and became a respected member of the Cuban-American community. She became a U.S. citizen in 1984.

Luis Zúñiga Rey, who was a political prisoner in Cuba before being expelled in 1988, said Wednesday that he met Juanita Castro during local radio interviews.

"She was serious but always kind and respectful," he said. "As the sister of the Cuban dictators, she always tried to keep her family background from interfering with her fellow Cubans here in Miami."

Her opposition to Fidel Castro "showed a lot of bravery," he said.

"Imagine what it is like to challenge your powerful brother and what that means at a personal level," he said.

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Fidel Castro ruled Cuba until 2008, when he turned power over to Raúl, who had been his second-in-command. Raúl Castro then spent a decade as Cuba’s leader.

When Fidel's serious health problems in 2006 led to street celebrations in Little Havana, she took no joy. He was still her brother, even though she fought against his government.

"In the same way that people are demonstrating and celebrating, I’m showing sadness. I respect the position of everyone who feels happy about his health problems, but they have to respect me also," she told The Associated Press. "It’s my family. It’s my brothers. It doesn’t matter. We are separated for political reasons, ideological reasons, but that’s it."

Fidel Castro died in 2016 at 90, while Raul, 92, is living in retirement. The eldest brother, Ramon, died in 2016 at 91.