updated 05/29/2011 AT 11:40 AM ET
•originally published 05/30/2011 AT 6:00 PM ET
“He fits in [with the family] already. I think they’re just themselves, and I think that’s the most important thing,” Kourtney, 32, told PEOPLE at Disick’s 28th birthday party at Las Vegas’s Gallery Nightclub over the weekend.
“Kim is totally herself in front of him,” she added. “Everything is easy, comfortable, no stress.”