In a sneak peek at Sunday’s episode, Kourtney, 37, reveals she lost her cool after Rob Kardashian‘s fiancée Chyna released a set of ChyMojis in May – and included one thinly-veiled jab at Kylie Jenner.
In the clip, Kourtney scrolls through headlines on her phone regarding the emoji in question, in which a girl closely resembling Jenner – with dark hair, defined eyebrows and plump, colorful lips – gets slapped across the face. (In case you’re not caught up, Jenner, 19, and Chyna, 28, share a long and complicated history largely stemming from the fact that Jenner is dating Tyga, 26, who was previously engaged to and shares a son with Chyna, who is now engaged to and expecting a child with Rob, 29. Still with us?)
Kourtney and family makeup artist Mario Dedivanovic analyze the emoji and decide it’s “totally” a depiction of Jenner – which Kourtney then explains “sent her over the edge.”
“Everything with Rob, I always try to be super calm but I was just like, in a mood,” she tells Dedivanovic. “So I was going to call Chyna, and then Kylie was like: ‘No, just call Rob, it’s too crazy to call Chyna.’ And I don’t want to yell at a pregnant person.”
“This is the first time that made me question Chyna’s intentions,” she adds. “I usually don’t let any of this kind of stuff bother me, but for some reason this just sets me off.”
Kourtney then explains she called her brother and yelled at him while Rob, for his part, fiercely denied Chyna’s emoji was aimed at Jenner.
“He was like: ‘What do you care if she’s doing this, everyone has emojis!’ ” Kourtney recalls. “And I don’t care that she has them, I care that it was Kylie.”
Kourtney’s sister Khloé Kardashian, 32, then calls Kourtney to discuss – what else? – the notorious emoji diss.
“I went off on him yesterday,” Kourtney tells her sister. “He was like: ‘I’m going to go on the red carpet tonight and I’m going to tell everybody how I have the worst, most unsupportive, disgusting family!’ Like, ridiculous. I think that it’s stupid to take it there.”
“I actually have a good tweet that I’m going to do,” Khloé says. “It’s subliminal.”
“Your subliminal tweets are not subliminal!” Kourtney replies, laughing.
RELATED VIDEO: Blac Chyna and Rob Kardashian Celebrate Khloe’s Birthday With Kim, Kourtney and Kylie
In the months since the scene was filmed, however, Rob’s famous family has warmed up to Chyna.
“The girls are making an effort to put any drama behind them and make Chyna feel welcome,” a source told PEOPLE after Chyna joined the family to celebrate Khloé’s birthday in June. “They’re trying to be as supportive as possible, especially now that she is pregnant.”
Keeping Up with the Kardashians airs Sundays (9 p.m. ET), and Rob & Chyna premieres Sept. 11, both at 9 p.m. ET on E!