The 36-year-old reality star and her 18-year-old half-sister spent time with the 32-year-old prior to singer Justin Bieber's 22nd birthday party Saturday.
"Reunited best friends lol," Kardashian captioned a photo of herself and Bailon sitting on Jenner's lap. Jenner also shared a snapshot on Instagram, writing, "Cause we are sistersss."
Bailon dated Kardashian's 28-year-old brother, Rob Kardashian, from 2007 to 2009. "The Real" host incited Kim Kardashian's wrath in 2014 by telling Latina magazine her relationship with Rob damaged her career.
"To be stuck with that Kardashian label, that was so hurtful to me and to my career," the star said.
"It's common knowledge that he cheated on me ... He strategically planned things out so that he could cheat on me, and that to me was so disloyal."
Bailon later told E! News she didn't mean to offend Kim with her comment and clarified she meant she "didn't want to be known as anyone's girlfriend."
She had nothing but praise for Rob while addressing reconciliation rumors in October.
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"I think Rob is an amazing guy," she said.
"I haven't gotten in touch with him or anything, but I think he's awesome and I wish him well. [The rumor] just isn't true. We don't [even follow each other on social media]."
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