Seventy-six-year-old Juanita Castro told Univision's WLTV-23 station late Sunday she initially supported her brother's 1959 overthrow of the Batista dictatorship but quickly became disillusioned.
Her new memoir was released Monday by Santillana USA.
Her home became a sanctuary for anti-Communists before she fled the island in 1964. In the TV interview, Juanita Castro says she was approached by the CIA.
The station said she collaborated with the agency while in Cuba and after she left. Those details are contained in a segment of the interview due to be aired Wednesday. A transcript was not available in advance.
Juanita Castro eventually settled into a quiet life in Miami and ran a pharmacy until 2007.