The Coming Clean star, who finished second in Channel 5's debut series of Celebrity Big Brother last month, predicted that the latest addition would "intimidate" the other housemates, especially her sister Faye.
"I think this is going to show Faye's true colours," Katona told Emma Willis on Big Brother's Bit on the Side. "I think Faye is going to be on her best behaviour now, massively.
"Jem is like a pure, mean bitch. I've not even met the girl yet, but I'm intimidated already. Faye is going to be completely intimidated... Jem completely dominates Faye."
Meanwhile, Mark Henderson, who voluntarily left the Big Brother house this week, approved of Jem's arrival, due to the potential for an explosive sibling rivalry.
"Anyone who can do the travelator deserves to be in there," he quipped, referring to Jem's former Gladiators role.
"[Faye is] going to be annoyed. If there's someone in there who's like you, but bigger and better, you'd be like 'Oh my God'."