The challenge by Adiela Diaz, sister of Palmira Oswaldo Diaz, came after FARC claimed it had stopped kidnappings for ransom while the group was in peace talks with the governments, Colombia Reports said Tuesday.
"If you say you are not holding any kidnapped people, then answer: what did you do with my brother," said Diaz during a radio interview, addressing her question to Jorge Torres Victoria, a FARC delegate at the peace talks.
"And if he is dead then admit it and put an end to this nightmare," she added.
Victoria released a video in 2006 in which he claimed Diaz, who had been abducted in 2001, was being held "prisoner" for "crimes of corruption."
FARC issued a directive last year banning "retention of persons for economic reasons."
The Colombian government estimated in June some 40,000 kidnappings had been committed in the country during a 40-year period ending in 2010. An estimated 37 percent of those abductions were allegedly carried out by FARC.