It's not very likely that their paths did cross.
Margaret Sinclair and Pierre Trudeau were married in March, 1971 and Justin Trudeau was born in November 1971.
I doubt she would have had time to fit a tryst with Fidel into her schedule.
Maybe she was thinking of Fidel when Justin was conceived, but it's highly unlikely that they got together to produce a child in 1971.
Okay, it would have been very hard for Margaret, the new toast-of-the-town First Lady, and a May-December bride to boot, to go anywhere without being followed by the paparazzi, especially just after moving in with the PM.
Perhaps the UK's "gay" facial recognition software could be adapted to those with autocratic socialistic tendencies, given the Trudeaus' and Castro's similar facial features?