08/12/2009 at 11:20 PM EDT
is going to be an aunt – her older sister Kourtney is expecting her first child.
"We were in the Everglades and I kept feeling nauseous and sick," Kourtney tells E!. "I just kept thinking something wasn't right. ... I went to the doctor and he confirmed the news. I was just so shocked."
She says the baby is due "right around Christmas time." Kardashian adds that her mother is "really excited." She didn't say who the father was.
The Keeping Up with the Kardashians
star, 30 – whose spin-off show with younger sister Khloe, Kourtney and Khloe Take Miami
, premieres Sunday – was previously linked to Scott Disick.
The on-again, off-again couple reportedly broke up in January, but have been spotted spending time together since then. The two hung out in the Hamptons in July, where they enjoyed dinner and, according to a source, Disick introduced Kardashian as his girlfriend when a pal stopped by their table.
News of Kardashian's pregnancy was first reported by E! Online
Reporting by MARK GRAY and JENNIFER GARCIA