Kylie, 18, and Tyga, 25, celebrated at a bash hosted by John Terzian and The h.wood Group for Kylie at Los Angeles hotspot Bootsy Bellows.
Kylie’s sister Kendall Jenner, 19, also attended the party, and a source tells PEOPLE the two arrived hand in hand alongside Tyga at about 12:30 a.m.
The source says Kylie and her beau stayed close to each other throughout the night, holding hands, kissing and cuddling.
“They were pretty much inseparable,” the source adds.
Kendall, who went pantsless as a super-sexy Karl Lagerfeld, “did her own thing, but definitely stayed close to Kylie and the table that they were sharing,” the source says.
The crew left a little before 2 a.m. but swung by Paris Hilton‘s table to give her a hug goodbye.
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The night before, Kylie donned a sexy eskimo/snow princess get-up, complete with sparkly makeup and a white wig.