Hathloul's brother wrote in an commentary for CNN on Thursday that his sister has been "whipped, beaten, electrocuted and harassed on a frequent basis."
"Whenever Loujain spoke about the torture sessions to my parents, her hands shook uncontrollably," he wrote. "I fear the pain will stay with her forever."
He added that he would "like to see [Carey] ask for the release of my sister while she is on stage." On Twitter, her sister Alia also asked Carey to remember that she is able to perform in Saudi Arabia thanks to Hathloul's activism.
"I wish she [could] attend your concert," she wrote. "But she's locked behind bars because she tried to improve women's condition."
But Carey has refused to back down. "As the first female international artist to perform in Saudi Arabia, Mariah recognizes the cultural significance of this event and will continue to support global efforts towards equality for all," her publicists said in a statement.
On Carey's Instagram account, commentators took both sides, with some urging her to cancel the show and others encouraging her to perform despite the criticism.
The plight of women in Saudi Arabia was in the spotlight in January thanks to Rahaf Mohammed, an 18-year-old Saudi woman who flew to Thailand on the way Australia, where she wanted to seek asylum. Her family was abusive, she said, and she feared she would be killed if she returned with them to Saudi Arabia. After Thai authorities detained her on her layover, she barricaded herself in a hotel room and demanded to speak to the U.N. refugee agency. Her story went viral, and she was granted asylum in Canada within days.