Kylie Jenner's birthday wasn't completely a family affair. The reality star's older brother, Rob Kardashian, was notably absent from his sister's 18th birthday celebrations over the weekend.

A source close to the family told Radar Online that the 28-year-old sock designer refused to attend her sister's birthday bash, adding that he has been ignoring his family and has not accepted any phone calls or replied to any text messages from his family.

"Rob refused to do anything for Kylie's birthday," the source said. "He is invited to everything and he refuses to come to anything. He's just ignoring everyone - his mom, his sisters. He just doesn't care."

Kylie, who turned 18 on Monday, celebrated her birthday with a dinner at Nobu on Aug. 7 and had a star-studded birthday bash at West Hollywood nightclub Bootsy Bellows.

The reality star's entire family came out to celebrate, including mom Kris Jenner and boyfriend Corey Gamble, Caitlyn Jenner, Kanye West, Kendall Jenner, Khloe Kardashian, Kim Kardashian, Kourtney Kardashian and several of Kylie's celebrity friends.

The insider added that Rob, who is currently living with his older sister Khloe Kardashian, refuses to leave his sister's Calabasas, Calif., mansion and Khloe is having trouble making him. Rob's friends are reportedly also considered about his behavior and "don't know what to do anymore." 

"He just hides out there all the time and doesn't care if he leaves or even gets dressed. Rob is really making his sisters and even his mom mad," the source added. "Kim still thinks he's really being selfish and she want him to do things with them, not shut them out completely."

In the new issue of Complex Magazine, Khloe opened up about Rob living with her, admitting that he's going through a "hard time."

"Rob lives with me now, and I wouldn't have it any other way. I love having him with me. I have a big ass house and I wanted it so he could feel like, comfortable," she told the publication.

"He's going through a hard time now. And [my home is] behind gates. With this crazy lifestyle you have to think of places where you can still have your freedom. Kourtney and Scott live in the same gated area. My nieces and nephew come over at least three times a week. I have everything, my brother's there and I love him."

Rob, who reportedly is dealing with his own insecurities after gaining weight, has not publicly addressed the press about his troubles.