Kamar Mahtum was travelling to London hoping to confirm her long-lost sister Siti Aishah Abdul Wahab, 69, was one of three women at the centre of a case that has shocked Britain.
Siti Aishah came to Britain as a high-flying student in around 1968 but turned her back on her family after joining the radical left.
The other women allegedly held by the couple are believed to be the daughter of a World War II code-breaker who also became a communist, and a 30-year-old who has spent her entire life inside the Maoist "collective".
Their alleged captors, named by media as Aravindan Balakrishnan, 73, and his 67-year-old wife Chanda, have emerged as radical communists who led a small Maoist splinter group in the 1970s.
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Relatives of the two older victims have described them as ordinary women who fell under the spell of the man known to activists as "Comrade Bala".
Siti Aishah was reportedly so drawn in by his Marxist rhetoric that she dumped her fiance and moved in with the collective.
The second woman has been named as 59-year-old Josephine Herivel, whose father John was one of the Bletchley Park codebreakers who helped Britain and its allies win World War II.
Raised in Northern Ireland's capital Belfast, Herivel is believed to have moved to London in the 1970s and, like Siti Aishah, disowned her family after becoming involved with the far-left.
Appearing in court in 1978 after the police raided the Maoist group's Brixton headquarters, Herivel shocked the judge by denouncing him as a "fascist lackey".