The 34-year-old had planned to celebrate with friends and family at a Troop Beverly Hills-themed party this weekend but has now axed the gathering.
A source told E! News: "She's totally focused on being there for Khloe right now."
Kim - who is expecting her second child with husband Kanye West – is currently in Las Vegas with Khloe, who has been by Lamar's bedside since Tuesday.
Former NBA player Lamar was found unresponsive at the Love Ranch brothel on Tuesday afternoon. He was taken to hospital, before being transferred to the Sunrise Hospital, where the Kardashian-Jenner family has been gathering.
No official statement has been released on his current condition but the brothel employee who dialled 911 claimed Lamar was not breathing and had been taking herbal Viagra. It's also been claimed he had cocaine with him.
All you need to know about Kim Kardashian: