Kylie Jenner Is 'So Jealous' of Kourtney Kardashian's Sizzlin' (& Naked!) Costa Rica Pool Photo Shoot

"I'm so jealous!" Kylie Jenner admits in the clip upon seeing her older sister's pool photo shoot

Kourtney Kardashian stripped down during her trip to Costa Rica.

In an E! clip of Sunday’s upcoming episode of Keeping Up with the Kardashians, the oldest Kardashian sister gets completely naked for a pool photo shoot one evening on the family vacation.

“So you really are getting naked?” Kylie Jenner asks Kourtney, 38.

“Yes! What do you mean?” Kourtney, who dons a robe, responds before the two engage in a “fake fight,” which includes the punching of a boob and the grab of a vagina.

After Khloé Kardashian joins her sisters outside and tells Kourtney to “focus,” Kourtney proceeds to take it all off.

E!

With Kim Kardashian West, Khloé and Kylie all watching from the sidelines and cheering her on, a naked Kourtney flashes smiles and smizes at the camera as she sits on a poolside rock and shows off her tanned and toned physique.

But it doesn’t stop there! The mother of three then moves the nighttime shoot into the swimming pool, where Kourtney practices her model poses — including the flip of the hair.

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“I’m just doing what I always came to do,” Kourtney quips about the racy session.

Upon watching the KUWTK star show off her birthday suit, Kylie admits that she’s a wee bit jealous — and wants to join in on the fun!

“What the f—? I want to get in,” Kylie, 19, says upon looking at the photos. “I’m so jealous!”

During their confessional, 32-year-old Khloé, who sits side-by-side with Kourtney, admits: “I honestly was worried about you after Penelope. I did think, like, She’s gone. … Now she’s YOLO-ing it up, f—ing bomb ass body, naked in the pool and your sisters should be afraid of you.”

Keeping Up with the Kardashians airs Sundays (9 p.m. ET) on E!