48-year-old Ruth López had long criticised authoritarian President Nayib Bukele for corruption and human rights violations when she was arrested in May last year.
After she was recently allowed to receive visitors for the first time, her sister said her condition is serious.
Her health has deteriorated significantly, says Claudia López in a video, warning that her sister is at risk of a heart attack if she does not receive adequate care.
Several hundred inmates have died during Bukele's war on gangs, most of them did not belong to any group, a report from March warned.
Since 2022, Bukele has maintained a state of emergency in the country, during which over 90,000 people have been detained without legal basis.
