Scott Disick reportedly cheated on partner Kourtney Kardashian with her younger sister Kendall Jenner, a new report from Star magazine claimed.

The Kardashian-Jenner clan were quick to react to the outlandish rumor, with Khloe Kardashian leading the charge, Daily Mail reported.

The 31-year-old E! star poked fun at the rumor, calling Disick out and claiming that she should be the one having an "affair" with him.

Kardashian wrote that she was "pissed" about the alleged cheating scandal, calling her older sister's partner a "THOT" and even referencing a Chris Brown song.

“These hoes really ain’t loyal!! Huh?!?! Damn Gina!!! Damn!!!” she tweeted after learning the news.

Jenner and Disick quickly joined in the fun, mocking the rumors along with Kardashian.

"Don't worry @khloekardashian, You will always be my number 1," Disick tweeted in response to Kardashian's post.

Jenner, pretending to be upset over the "betrayal," chimed in, tweeting: "Excuse me?!"

The Star report (via Hollywood Life) alleged that Disick, 31, had sex with Kardashian's younger sister, 19-year-old Kendall Jenner, while his partner was pregnant with their third child, Reign, whom they welcomed December last year.

Disick and Jenner have been seen hanging out together numerous times, usually going shopping and eating lunch together. This doesn't come as a surprise though, as Disick has also been spotted out together with Kardashian's other sisters.

However, the latest cheating rumor plaguing the couple alleged that Kardashian has noticed Disick "checking out" Jenner several times and this has driven a wedge into their relationship. The shocking allegations also claimed that Disick has become a "corrupter" to Jenner instead of "protective" older brother.

“After their partying episodes, he saw Kendall as an adult,” a source claimed to Star magazine. “A superhot and fun one at that.”

“This is what their relationship has turned into," the same insider continued. "What should be a brotherly/sisterly love has become sexualized. Scott clearly finds her attractive, and she likes the attention. It’s just wrong.”

According to the report, Jenner and Disick allegedly hooked up at a party and Kardashian recently found out about the affair through a mutual friend.

Kardashian has reportedly broken things off with Disick and isn't speaking to her younger sister, insisting "they're dead to me" when she discovered the affair.

“At first, Kourtney was in complete shock and disbelief. But after she processed the information, she was devastated … She started screaming at [Scott], calling him ‘sick.’ But he kept denying it," the source claimed.

"He stormed out and the next she heard of him, he was doing what he always does — drinking his problems away. This time it wasn’t in Las Vegas, it was in Aspen.”

Meanwhile, Jenner's mom and the Kardashian matriarch, Kris Jenner, is reportedly "worried" over what this latest revelation would mean for the future of her grandkids: Mason, Penelope and Reign.

“She confronted Kendall about it," the source claimed. "But Kendall denied everything, then accused her mom of being happy about the rumors because they will raise her previous TV ratings.”

Well, judging by how quickly the Kardashians dismissed the reports, it's safe to say that there's no truth to the allegations.