NEW PIC: Glum Kim Kardashian Chills Out at Kendall Jenner’s Sweet 16

 Michael Simon/

Guess she didn't want to say cheese.

Kim Kardashian joined her entire family on Saturday night for little sister Kendall Jenner's Sweet 16 bash at the Andaz Hotel in Hollywood — and she was happy to cede the spotlight her fledgling model sibling, who was gifted with a 2012 black Range Rover.

In a new pic from the party, the 31-year-old reality star (wearing a beige top and black leather pants) didn't crack a smile as she sweetly posed with the birthday gal plus sisters Kourtney and a similarly glum Khloe Kardashian.

Tweeted Kim: "Celebrated @KendallJenner's Sweet 16 tonite w our family & friends! I remember my sweet 16! It was the perfect nite! She will never 4get it!"

An insider adds that Kardashian, who filed for divorce from Kris Humphries Oct. 31 and filmed The Marriage Counselor with Tyler Perry in Atlanta last week, was "nice to fans and took pictures at the party."

Other attendees included Kendall's youngest sister Kylie Jenner, Rob Kardashian, Kourtney's beau Scott Disick and parents Bruce and Kris Jenner.

Filming her movie in Atlanta, Kim was similarly low-key. "She's being very quiet and isn't smiling very much," a set source told Us Wednesday, adding that she was seemed very "on task" for the job at hand. "Kim has been reading a script and not being very social at all."

Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!

Want stories like these delivered straight to your phone? Download the Us Weekly iPhone app now!