While Kim Kardashian and Kris Humphries celebrated their engagement in Monaco, her sister Kourtney marked another milestone in Las Vegas: Her baby daddy's birthday.

Kourtney's man, Scott Disick, turned 28 on Thursday and to mark the occasion, the duo went to Sin City to celebrate. Dressed in a tight, sparkly cream dress, Kourtney attended Scott's b-day bash Friday night at Chateau Nightclub at Paris Las Vegas -- hitting the red carpet together before sharing cake inside.

The two also hit up Sugar Factory American Brasserie, where they feasted on some giant cupcakes.

Friday wasn't the only party for Scott though, as he and Kourtney kept the celebrations coming all weekend -- Kardashian tweeting Sunday "Heading home from a night out in the Hamptons at Dune with @scottdisick. Birthday celebration #4 lol. Can't hang like I used to. #gettingold."

Check out their Vegas night out above!

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