Kim Kardashian is "getting right for the new year" work-out wise, and recently her sister Kendall Jenner Instagrammed a sexy selfie of her showing off her booty at the gym, which Jenner decided to photobomb and show off her own toned butt, E! News reports.

"Barging in on Kim's booty selfie," the 18-year-old "Keeping Up With the Kardashians" star captioned the pic of her and famous sister. Both ladies wore stomach-revealing sports bras and tops, tight work-out pants and sneakers for their gym outing, posing in front of the mirror and showing off their svelte physiques.

Since giving birth to her daughter North West last year, 33-year-old Kardashian has been on a strict work-out and diet plan, carefully following the low-carb Atkin's diet to lose her baby weight and get her body back in tip-top shape. On Friday, the famous mama took to Twitter to slam rumors that she had undergone surgery for weight loss and photoshopped her latest post-gym pics.

"It sucks when people make up surgery or photoshop lies when I am so disciplined & work so hard," she told her fans during a Twitter chat. "Just trying to motivate others & show anyone struggling with weight they can totally achieve whatever they want if they are dedicated!"

Perhaps Kardashian is working extra hard to prepare for her upcoming wedding to Kanye West, rumored to take place sometime this year. Meanwhile, Jenner is allegedly dating One Direction's Harry Styles.