Kylie Jenner reportedly skipped older sister Kendall Jenner's recent birthday celebration because of jealousy.

According to RadarOnline, things have become quite awkward between the Jenner sisters lately, as they rival for fame. This reportedly explains Kylie's absence at Kendall's birthday lunch on Monday.

"It is a competition between them now as to who can be the more famous Jenner, and it is destroying their relationship," a source close to the Kardashian clan revealed to RadarOnline. "It's sad. They used to be so close and now they are really letting this fame thing affect them."

Kylie is apparently trying to break out of supermodel Kendall's shadow, as she is currently focusing on becoming a pop star, taking vocal training from producer Tim Carter, who previously worked with Beyoncé and Jay Z and created Willow Smith's hit "Whip My Hair."

"Kylie is now working on her first single," another insider claimed. "She is telling everyone that she knows she is going to be the next Katy Perry. ... She realized that she can't compete with Kendall when it comes to modeling. ... So she has chosen to turn to music."

According to the source, Carter has gone to mother Kris Jenner's house to give Kylie lessons.

"She thinks that her music career is going to take off and says she is getting her name out there right now. ... But it's just another way of her trying to one-up Kendall," the source continued.

The source added that Kylie basically replaced Kendall's spot with half-sister Khloé Kardashian.

"Kylie feels like Kendall abandoned her when her modeling career took off, and she kind of did," the source explained. "That's why Kylie has gotten so close to Khloé recently. Khloé is the only one that keeps it real anymore."