Miami, Florida - Juanita Castro, the younger sister of Fidel and Raul Castro, died on Monday in Miami where she spent decades in exile fighting for the downfall of her brothers, a journalist close to the family said.

"Today Juanita Castro, an exceptional woman (and) tireless fighter for the cause of the Cuba she loved so much, passed away," Maria Antonieta Collins, coauthor of Castro's autobiography, posted on Instagram.

Juanita Castro, who was 90 at her death, fled Cuba in 1964 after breaking with her brothers Fidel and Raul over the course of the 1959 Cuban Revolution.

She took up exile in Miami, where she regularly flayed her brothers and even collaborated with the CIA, using the codename Donna, in its efforts to topple the Castro government, she said publicly.

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The fourth child among seven siblings, Castro opened a drugstore in south Florida where she worked for decades.

She found it challenging to balance her dual roles as both sister of the Castros – widely reviled in Cuban communities in south Florida – and member of the exile community, as she recounted in her memoir, My Brothers Fidel and Raul, the Secret History.

"Undoubtedly, I have suffered more than the rest of the exiles because nowhere along the Straits of Florida do they give me respite and few are those who understand the paradox of my life," she wrote. "For those in Cuba I am a deserter because I left and denounced the regime in place. For many in Miami I am 'persona non grata' because I am the sister of Fidel and Raul."

According to Univision, Juanita Castro died of natural causes in a Miami hospital.

Her older brother Fidel died in 2016 in Havana, while Raul Castro, who is now 92, stepped down as head of the communist party on the island in 2021.

"According to her wishes, her funeral will be private," Collins wrote in her Instagram post.