October 01, 2015 03:35 PM

On this season of Keeping Up with the Kardashians, Khloé Kardashian has fiercely defended her relationship with ex Lamar Odom to sister Kim Kardashian West – and the upcoming episode is no exception.

In a sneak peek at Sunday’s episode, Kardashian, 31, and Kardashian West, 34, are at Kardashian’s home when she receives a call from Odom.

Though she says, “I’m with Kim, can I call you back?” immediately upon answering, the two end up chatting for some time, with Odom even asking his ex to send his regards to Kardashian West, her husband Kanye West, 38, and their daughter North, 2.

As Kardashian steps outside to continue the phone call, her older sister purses her lips and crosses her arms.

In an on-camera interview, Kardashian West says, “I don’t know why Khloé and Lamar are talking, but he hurt her so badly.”

“As a big sister, I’m just really protective and for her just to be flirting with him on the phone makes me like, sick to my stomach,” she continues.

“Kim is staring at me, I have to go,” Kardashian says to Odom on the phone as she feels her sister’s watchful stare.

“Like, I just can’t take it,” Kardashian West says once Kardashian has hung up. “The fact that when he decides to come around and be friendly, you’re like, flirting with him.”

“I’m flirting with him how?” Kardashian retorts. “By answering the phone and saying hello?”

“You’re just like, way more attentive,” Kardashian replies, adding that her sister would be much more dismissive if the phone call had come from her other ex, rapper French Montana.

“Why would French and Lamar even be in the same category?” Kardashian responds.

Kardashian and Odom wed in 2009 just one month after meeting. After he relapsed with his substance abuse problems and cheated on Kardashian, she filed for divorce in December 2013. The exes finally signed the divorce papers July 21.

Odom allegedly ambushed and verbally attacked his ex-wife outside a SoulCycle studio in Beverly Hills, California, in August.

Though the former basketball player fiercely denied the reports, a source close to Kardashian told PEOPLE at the time that the reality star was “emotionally shaken.”

In the wake of the incident, Kardashian spent time with her current boyfriend NBA player James Harden. On Aug. 24, she celebrated the NBA player’s lavish 26th birthday bash aboard a yacht with the rest of her famous family.

A source close to Kardashian told PEOPLE in August that Odom “has issues” with his ex-wife dating Harden.

Another insider told PEOPLE Odom “had a hard time letting go of Khloé” but that he is “healthy and not drinking or doing drugs.”

Just last week, Odom returned to Instagram and posted a healthy food snapshot.

Kardashian’s relationship with Odom has been a source of contention on this season of KUWTK, which was filmed before the alleged altercation between the exes took place.

On a recent episode, Kardashian told Kardashian West to "take a chill pill, slore," when her sister questioned Kardashian for checking in on Odom during a trip to Armenia.

Keeping Up with the Kardashians airs Sunday (8 p.m. ET) on E!

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