Kendall Jenner and Hailey Bieber Celebrate Billie Eilish's 21st Birthday in Photo Booth Pics

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Kendall Jenner and Hailey Bieber let their hair down on Sunday for Billie Eilish's 21st birthday party.

The supermodels were slightly misbehaving with tongues out and middle fingers blazing in black-and-white photo booth snaps at the Christmas-themed bash. On Jenner's Instagram Stories, she showcased solo shots of the three pals and a couple silly pics sporting various sneers while Bieber gave a kissy face.

Eilish was dressed as Mrs. Claus in a sexy Santa suit. She had a three-tier cake decked out with peppermint candies for the milestone affair, which was lined with disco balls hanging from the ceiling.

Kendall Jenner Instagram

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Per videos shared on Eilish's own Instagram Stories, guests raised their glass for the "Bad Guy" singer in a birthday toast. Her boyfriend, Jesse Rutherford, and Hailey's husband, Justin Bieber, were also in attendance, along with Doja Cat, Avril Lavigne and Noah Cyrus.

Jenner, 27, appeared to be flying solo for the night. Several sources confirmed to PEOPLE last month that the Kardashians star and Phoenix Suns guard Devin Booker "quietly" ended their relationship in October due to their demanding jobs.

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"Both have incredibly busy schedules right now with their careers and they've decided to make that a priority," a source tells PEOPLE. For Jenner, 27, that includes her 818 tequila line and her booming modeling career, and for Booker, 26, it's his involvement with the NBA.

"They have a lot of love and respect for each other and wish only the best," another source added.

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As for Hailey, 26, she and her longtime love appear to be going strong, constantly supporting one another on social media. Justin, 28, announced in June that he has a facial paralysis condition called Ramsay Hunt syndrome and Hailey suffered from a mini-stroke in March.

While doing press for her new skincare line, the YSL model opened up about her scary episode and Justin's recent diagnosis.

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"We both are dealing with older illnesses for sure," she joked on Live with Kelly and Ryan, before getting reflective. "Honestly the silver lining in the whole situation — what he went through, what I went through — is it really bonds you. I feel like we're closer than ever. It really brings us close. So I'll take that away from it."

She echoed the sentiment on Good Morning America, adding that having to go through health scares publicly pushed the couple closer — and to be open with one another about everything. "It in a way forces you to have to just be upfront about what's going on so that people understand what you're going through," Hailey said. "I actually think it opened up a lot of really important and amazing conversation."