Relive Kourtney's 40th Birthday Blowout

"I don't think Kourtney really understands the legwork that goes behind this," says a frustrated Kim.

Kim Kardashian feels her older sister, Kourtney Kardashian, copies her style -- but Kourt begs to differ.

In a sneak peak of Sunday's episode of "Keeping Up with the Kardashians," the sisters get into a fashion-driven fight when Kourtney tries on a dress for Khloe Kardashian, who insists on FaceTiming Kim to get her opinion.

The sparkly-pink vintage Dior mini-dress Kourt had on was one of many outfits she was considering for her upcoming 40th birthday bash. She liked it but felt it was a bit too "Romy and Michele," so she wanted a second opinion (third if you count Khloe's).

Kourt wanted Khloe to FaceTime the model in the family, Kendall Jenner, because she wasn't sure if she "trusted" Kim's opinion. Khlo called her anyway.

"She wants me to call Kendall because she doesn't care about you and I," Khloe said to Kim after showing her Kourtney's look.

"Well, if you don't care about [Khloe] and I, why do you keep on picking out all the same clothes?" Kim asked her older sis. Khloe's eyes widened.

"I don't know what you're talking about! I've never dressed like you," Kourt fired back, as Khloe cowered back into the bed she was sitting on. "Oh, is there a fight here?" she whispered to herself.

"But Danielle was like..." Kourt continued, prompting Khloe to warn her that Danielle was with Kim. "I don't care if Danielle's there. Hi, Danielle!" Kourtney continued. "I dress completely different than Kim!"

"[Thierry] Mugler was here. He said Dani sent the longest message..." Kim tried to reply, but Kourt cut her off. "Dani told me today that she sent that email. I had no idea she sent it, and she said she sent it because she knows we have completely different styles."

"I'm not trying to copy your looks," she maintained, "but on the same designer, you can't ban me from Rick Owens when I wear mens Rick Owens."

"No, they said, 'Sorry, Kourtney's stylist pulled everything..." Kim tried to reply, but Kourtney interjected, "So ask me! Say, 'Hey, are you wearing this white Rick Owens dress? I wanted it.' She might pull things, and then when it comes here, I say, 'No, no, no, no, no. Yes, yes, yes, yes, yes.'"

"Let's call Kendall," Kourtney said to Khloe," who I actually care about her style."

"It's so ridiculous that we're even having a conversation like this because Kim and I have completely different styles," Kourtney later vented during a confessional. "So even if we wear the same designer, we wear it in a completely different way. The store may send me clothes that are more Kim's style, and I send them back."

In her own confessional, Kim explained that it's about more than just her sister having a similar look.

"I don't think Kourtney really understands the legwork that goes behind this. Like, I don't think anybody understands. You have to really have a relationship with that designer. I've spent months and months flying to Miami to see what [Christian Dior's] vision is, so for her to jump in, I just was like, 'Oh, come on. She has no idea how this works.'"

"Keeping Up with the Kardashians" airs Sundays at 9 p.m. on E!

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