Khloe Kardashian has slammed her sister Kourtney's relationship with former partner Scott Disick as ''dysfunctional'' and ''f***ing nuts''.

The 38-year-old reality star ended her tumultuous on/off relationship with Scott - with whom she has three children, Mason, seven, Penelope, five, and Reign, two - for good in 2015, but in a new preview clip for their reality show 'Keeping Up with the Kardashians', younger sister Khloe isn't convinced the pair are really over.

After Scott, 34, confronts Kourtney - who is now in a relationship with Younes Bendjima - about not being invited to Khloe's surprise birthday party, the 33-year-old beauty steps in to comment on their relationship.

Speaking in the clip, Khloe tells the pair: ''You guys are so f***ing dysfunctional. I don't know if this is some thing you guys do as foreplay but this is f***ing nuts.

''You two are so f***ing nuts seriously. And the fact that you waste my time when you guys are now just gonna go get mani [or] pedis.''

Kourtney then insists that she and Scott - who has since moved on with 19-year-old Sofia Richie - no longer ''hook up''.

But Khloe adds: ''You guys are f***ing sick!''

In a previously shared clip of the upcoming episode - which will air on E! on Sunday (12.11.17) - Scott hit out at Kourtney for not inviting him to the family bash.

He said sarcastically: ''F**k, I didn't even get invited to her birthday. And I'm family. That was really sweet. That made me feel really good ... I highly doubt that Khloe was like, 'The last person I'd want at my surprise party would be Scott.' It's cool. Honestly, I called my sister and I hung out with her the whole night.

''And then, actually, I called my mom after and was [like], 'Hey, mom. Do you want to come over? Because I'm feeling a little down and lonely because the family that claims I'm still blood even though we're not together still doesn't invite me to a surprise party.' But I really felt good about it.''

However, Kourtney defended her decision to keep Scott off the guest list.

She said: ''I didn't want to have you coming to the birthday and be photographed going in. I'm a little bit caught off guard. I was trying to be positive. I didn't want to say I just didn't want you there. So, if I can't say anything nice, I just won't say anything at all.''