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Kourtney Kardashian, Kendall Jenner

Jeff Kravitz/FilmMagic

Saturday was sisters' night out for Kourtney Kardashian and Kendall Jenner, who were each other's dates at The Comedy Central Roast of Justin Bieber.

Kourtney, 35, looked gorgeous in a black and white mini dress with a tuxedo design, which showed off her slim post-baby body perfectly, while her 19-year-old model sister turned heads herself in a ribbed, gray, off-the-shoulder top and black knee-length pencil skirt, paired with gladiator sandals.

Kourtney, who three months ago gave birth to son Reign Aston Disick, her third child with partner Scott Disick, recently revealed her weight via an Instagram photo, saying it was the lowest figure she has seen in a while. The Keeping Up With the Kardashians star, who has usually appeared slim, had started working out at the gym about a month after her pregnancy.

Kourtney Kardashian

Jeff Kravitz/FilmMagic

Kourtney Kardashian, Kendall Jenner

Jeff Kravitz/FilmMagic

At the Bieber roast, she and Jenner were photographed sitting in the front row of the Los Angeles theater where the taping took place and speaking toMartha Stewart who was one of the roasters.

Kourtney Kardashian, Kendall Jenner, Martha Stewart

Christopher Polk/Getty Images

The sisters also attended an after party, along with fellow celebrities such asJaden Smith, and Jenner chatted with the Biebs himself. The two have hung out several times before.

The event took place a day before the season 10 premiere of Keeping Up With the Kardashians. The Bieber roast will air on Comedy Central on March 30. Kevin Hart was the roast master, while other roasters included Jeffrey Ross, Snoop Dogg, SNL's Pete Davidson and Hannibal Buress.

Bieber called the roast "one of the best nights ever."

Kourtney shared several selfies and other photos from the event on Instagram. Check them out below.

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A photo posted by Kourtney Kardashian (@kourtneykardash) on

Bad and it shows.

A photo posted by Kourtney Kardashian (@kourtneykardash) on

No caption necessary.

A photo posted by Kourtney Kardashian (@kourtneykardash) on

My obsession with these people is not normal! #justinbieberroast

A photo posted by Kourtney Kardashian (@kourtneykardash) on

The season 10 premiere of Keeping Up With the Kardashians begins on Sunday, March 15 at 9 p.m. ET on E!