Kim Kardashian (left) and Kendall Jenner
Courtesy of Kendall Jenner
01/11/2014 AT 01:40 PM EST
That is one booty-ful photobomb.
jokingly showed off her, um, assets
Friday when she sidled up next to big sister Kim Kardashian
's famous derriére.
"Barging in on Kim's booty selfie," Jenner, 18, captioned the Instagram shot
she posted early Friday.
The duo certainly look ready for a workout: The sisters are both sporting figure-hugging black workout pants and sports bras. Kardashian, 33, is also wearing black Nike sneakers, while the flash blocks out her face.
and grueling workouts
are nothing new for Kardashian, who has been working on her post-baby body ever since welcoming daughter North
with fiancé Kanye West
She also recently made it clear that her hard work at the gym is responsible for the return of her famous figure – and nothing else.
"It sucks when people make up surgery or photoshop lies when I am so disciplined," she wrote on Twitter
Also not new for the reality star? Appearing in amusing photos with family and friends, as evidenced by this sweetly dorky Throwback Thursday photo released this week
, featuring Kim and pal Nicole Richie