Eleri Morgan has told ITV News her cousin Sian Davies lived within the collective led by slavery suspect Aravindan Balakrishnan in Brixton.
She explained Sian wrote letters home, talking of how she was "looking after the mothers of the world" but when Eleri attempted to visit her she wasn't allowed to see her.
She added: "We knew they were keeping her away from us...And the only time she visited my Auntie, she was with two minders."
Documentary footage of the man said to have kept three women as slaves in south London has been uncovered by ITV News.
The political past of a couple accused of holding three women against their will for more than 30 years, has emerged.
A couple were arrested after police arrested three alleged slavery victims said to have been held against their will for at least 30 years.