Kourtney Kardashian: Kim Is Herself Around Kris Humphries

By Mark Gray

05/30/2011 at 06:00 PM 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."

PHOTOS: Kim & Kris's Whirlwind Romance

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."