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From the Wikipedia listing for Justin Trudeau's mother Margaret Trudeau, who married Pierre Trudeau about the time she became pregnant with Justin. Was this unidentified island that the Trudeau's visited Cuba?
Also, this article from the Ottawa Citizen: Margaret Trudeau's Cuban memory: The dictator who cuddled her baby talks about Margaret Trudeau and Fidel Castro's relationship. It almost sounds like Fidel was also the father of Justin's younger, now deceased, brother Michel Trudeau.

Fidel Castro was immediately taken with four-month-old Michel when he arrived with his parents in 1976, Margaret Trudeau remembers. Fidel Castro was immediately taken with four-month-old Michel when he arrived with his parents in 1976, Margaret Trudeau remembers. The Ottawa Citizen
...
Margaret Trudeau said she visited Castro at his hotel before the funeral to welcome him on behalf of the family.
The Cuban dictator had an ongoing fondness for the Trudeaus’ youngest son who had charmed him as a baby. Castro was in tears, Trudeau said, when he learned of Michel’s death in 1998 in a British Columbia avalanche.
Trudeau said Castro had wanted them to send Michel back to Cuba when he was seven to attend pioneer camp for children, something the family often teased Michel about.

Margaret Trudeau stands with baby Michel next to Fidel Castro and Pierre Trudeau at the departure ceremony in Havana at the end of the Trudeaus' 1976 state visit to Cuba. Photo by Rod MacIvor /Ottawa Citizen