Television host Poppy Jamie, who also attended Jenner's celebration, shared a video from the reality star's birthday party in which Delevingne and Gomez appear singing a Happy Birthday duet.
"I don't know if it's even a party, but a girls' gathering," she said. "It will be fun!"
Asked how she felt about her younger sister getting older, Kardashian said Jenner was both old and young in her eyes.
"It's crazy because she's still so young, but then at the same time I'm like, 'She's so old because she's done so much,' especially in the past year," she said. "I'm so proud of her, and she's still such a good girl. She's just a goofy girl and I love that about her. I want her to stay that way."
While Jenner has yet to share any pictures from her party, she did take to Instagram to post a photo from several of her gifts.
"love on top of LOVE. so much love today!!!" she wrote in the caption.