It's hard trying to keep up with all of the Kardashian rivalries! When Keeping Up With the Kardashians premiered nearly a decade ago, Kim Kardashian was definitely an "it" girl, standing out as the show's star. But now her youngest sister Kylie Jenner has come into her own as a celebrity — she built a mini empire with her lip kits and stars in her KUWTK spin-off Life of Kylie — and sources exclusively reveal to Life & Style that Kim is definitely not happy about being de-throned.

“[Kim is] privately jealous over Kylie’s success because she always thought she’d be the first Kardashian billionaire,” an insider says. “There’s a real rivalry between the Kardashian girls.”

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The 36-year-old is especially feeling slighted after her mom Kris Jenner was interviewed alongside Kylie by Women's Wear Daily about the massive success she's had with her makeup line, Kylie Cosmetics. Since its launch in November 2015, the company has earned $420 million — and it's on track to become a billion-dollar company by Kylie's 25th birthday.

“When Keeping Up With the Kardashians first started, all eyes were on Kim, and Kylie was a lesser presence because she was so young,” the insider notes. “Now Kim is feeling the competition, and it bothers her. She’s told those close to her that the Kardashian brand wouldn’t be where it is today if it wasn’t for her.”

Kim was so jealous watching her sister earn the title as a makeup mogul at just 20 years old, that she decided to launch her own makeup line, KKW Beauty, but it looks like sibling rivalry runs in the Kardashian family! While filling in on co-hosting duties on the Monday, Aug. 28, episode of LIVE with Kelly & Ryan, Kim opened up about her daughter North West's intense rivalry with her son Saint.

"I mean, I don’t know if it’s because she’s the older sister — I don’t know what it is. I thought it was a phase — she does not like her brother, it’s so hard for me," Kim admitted. "I thought it was like, 'OK, a couple months, she’s just warming up to it.'"

For more on the Kardashians, pick up the latest issue of Life & Style, on newsstands now!