Kim Kardashian is rumored to be disgusted by older sister Kourtney Kardashian's reported dalliances with bad boy Justin Bieber and has not been shy about letting her know it.

"Kim has told Kourtney she's a disgrace and it's lame and disgusting," a source told Radar Online of the rumored fling between the 36-year-old mother of three and Bieber, 15 years her junior.

A source added, "Kim has been begging Kourtney to think of the kids instead of constantly trying to get back at Scott Disick with all these 'look at me' outings and sleazy late nights with Justin and other young kids."

Back on Dec. 12, Kourtney Kardashian was seen leaving Bieber's Beverly Hills residence at The Montage in the early morning of the hours. Five days after that she was spied openly flirting with him during a dinner date, also in Beverly Hills.

A source described Kim Kardashian as "grossed out" by it all and surprised to see such behavior coming from her big sister.

Kourtney Kardashian split from 32-year-old Disick earlier this year, after he was spotted cavorting with ex-girlfriend Chloe Bartoli in Monaco over the July Fourth holiday weekend. Since then, Disick has also been romantically linked to teen model Lindsay Vrckovnik, and the two spent part of the summer jetting across the country together, hitting the party scene.

More recently, Disick spent a period in a Malibu rehab facility, and he has expressed an interest in reuniting with the "Keeping Up With the Kardashians" star and his girlfriend of nearly a decade.

The two were once thought to be working on ironing out their differences, after Kardashian and Disick were spotted out and about across Los Angles together.

But since Bieber entered the equation, Disick is rumored to have been growing increasingly angry about the troubled nature of their relationship.