Angola enjoyed the backing of Cuban leader Fidel Castro during the 27-year civil war
Late Cuban leader Fidel Castro (on the left)

Juanita Castro, the younger sister of the Cuban leaders Fidel and Raul Castro, has died at the age of 90 in Miami, Florida.

She died of natural causes at a hospital in Miami, according to a statement posted on Instagram by journalist María Antonieta Collins on Monday.

"Today, at 90 years of age, Juanita Castro went ahead of us on the path of life and death, an exceptional woman, a tireless fighter for the cause of her Cuba," read the statement translated from Spanish.

"There will be no interviews and the funeral will be private. We ask for your prayers for the eternal rest of her soul," she said. Juanita fled Cuba in 1964; however, she became a staunch anti-communist soon after the 1959 Cuban revolution.

In her book that she co-wrote with Collins, Juanita revealed that she had begun collaborating with the CIA shortly after the US' failed Bay of Pigs invasion in 1961.

"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 in her book: "My Brothers Fidel and Raul, the Secret History."

"For those in Cuba, I am a deserter because I left and denounced the regime in place. For many in Miami I am a 'persona non grata' because I am the sister of Fidel and Raul."

Juanita lived in exile in Miami for decades after a family row in the 1960s. The row led to a huge rift between the siblings after her brothers decided to give away the family's plantation. Fidel called his sister a "counter-revolutionary worm" after she sold off some livestock belonging to the family.

Juanita had even worked with the CIA against Fidel Castro's communist government. Fidel ruled Cuba until 2006 when he eventually transferred power to his brother Raul due to his ill health.

Fidel Castro was the son of a wealthy sugar planter. He led a guerrilla campaign with popular support and ousted right-wing dictator General Fulgencio Batista in 1959. He launched a political, social, and economic revolution that transformed the Caribbean island. He later turned Cuba into a communist state.

While Castro's supporters hail him as a revolutionary leader who has returned Cuba to its people since coming to power in 1959, critics call him an authoritarian. He died at the age of 90 in 2016. Juanita did not attend his funeral, but she did express her grief upon hearing the news.