Kourtney Kardashian and Scott Disick
05/29/2011 at 11:40 AM EDT
With Kim Kardashian
and fiancé Kris Humphries
due to head down the aisle
, her sister Kourtney Kardashian and Scott Disick say that Humphries brings out the best in Kim.
"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."
The same may be said for Kourtney and Scott, who find it just fine being an unmarried couple.
"We're happy," Disick says. "I think that if it's not broke don't fix it. We couldn't be happier."