Stephanie Pratt, the younger sister of Montag's husband Spencer – who is enduring the Costa Rican jungle alongside Montag on the four-nights-a-week show – is claiming Montag's health was pushed to the absolute brink.
"Heidi was virtually on her deathbed," Pratt, 23, told PEOPLE Sunday at the Elizabeth Glaser Pediatric AIDS A Time for Heroes Celebrity Carnival in Los Angeles. "She's never been sicker in her life. I can't believe NBC did that."
Pratt says she spoke to her brother over the weekend. "He was calling my mom until like 5 a.m. with updates about Heidi," she said. "They are just in the black rooms with no food or water and freakin' spiders biting them."
But a show insider contradicts Pratt's claims.
"The accusation that the couple was harmed in any way is completely false," the source says. "The room was not filled with critters and they were protected from the elements, unlike the other contestants, who endured some strong thunderstorms. When they came out, they were in good spirits ... No one at any point was held against their will. Everyone who is there wants to be there."
Montag and Pratt have already tried to leave the set of I'm a Celebrity ... Get Me Out of Here! more than once since its premiere last week.
On Thursday, Pratt and Montag were put in show's Lost Chamber as a punishment for leaving and then asking to return. While Stephanie Pratt says the couple was in the chamber for 72 hours, a source says the couple was only there for 14 hours, from 4:30 p.m. Thursday to 6:30 a.m. Friday.
The next day, Pratt says, Montag was admitted to a Costa Rican hospital.
"Basically, she started throwing up," Pratt explains. "She threw up like 30 times because she had no food in her. I guess she went unconscious and was rushed to the hospital with an IV in her arm. I understand [that quitting the show] was not cool ... It was pretty awful. But the punishment they got went way too far."
Another insider tells PEOPLE that Pratt is exaggerating. "There was no torture. Heidi and Spencer were covered with a ceiling and walls while their castmates have been through thunderstorms every night," the source says. "They were not stuck in there for days. Stephanie is talking like they were locked up like prisoners at Guantanamo Bay."
NBC has no comment.
The show's production company, however, Britain's ITV Productions, refuted claims the couple was deprived of food or water: "All allegations of the celebrities being deprived of food and water are completely untrue," the company said in a statement.
REPORTING BY ELIZABETH LEONARD