Their marriage ended more than three years ago in bitter divorce, but Karl and Cassandra have put their differences aside to combat "outrageous" claims from her half-sister Allegra over a nasty family spat.
A close friend tells Woman's Day both Cass, 49, and Karl, 45, were both rocked by the false reports in a tabloid newspaper from Allegra, who claimed their marriage was a "war zone" and that Cass was "obsessed" with Karl's second wife Jasmine.
"Throughout their marriage both Karl and Cass were nothing but generous and kind to Allegra," the friend says.
"They took her on holidays and went out of their way to make her feel a welcome member of the family.
Allegra claims Cass enlisted her to help her monitor news feeds about Jasmine and Karl when they wed in Mexico in 2018 and she was staying with Cass – but at the time Allegra was studying at university in Melbourne and could not have done this.
Cass refuted her half-sister's claims on her own social media in a blistering response alleging the feud was caused by Allegra's bad behaviour as their father lay dying of cancer 18 months ago, but she has since removed the post.
"Karl and Cass have endured a very bitter split, but despite all the scrutiny, Karl would be feeling for Cass after seeing Allegra's bizarre story - he knows how much Cass went above and beyond to help this troubled young woman," the friend says.
WATCH: Cassandra opens up about her high profile divorce
What's more, Allegra says she stayed at Cass's house when Karl and Jasmine married adding that she was asked to monitor the news feed and report back while Cass was on the school run.
"She just wants to know what people are saying," she told the publication.
"That is how she feels she is in control." Allegra also remarked that the divorce "changed her as a person" but in a now deleted Instagram post, Cass categorically denied her half-sister's claims.
Cass and Karl parted ways in 2016 and share three children Jackson, 20, Ava, 14 and River, 12.
Speaking to Woman's Day in 2019, eldest son Jackson said, "My mother is the most extraordinary person, and what she has weathered these past few years, no one deserves the backlash."
"To watch my parents' private lives play out so publicly was incredibly destructive for all of us – someone had to step up and protect her, and I knew that person was me."