Despite posing for a photo with pal Faye Resnick at Thursday's Kardashian-Jenner Christmas Eve party, sources tell PEOPLE the sisters are still very much at odds.
"There is still lots of tension," says the insider. "There is no truce."
Adds the source: "They put on a good face at the party [but] there's a lot to work on still."
A source revealed exclusively to PEOPLE that the Real Housewives of Beverly Hills star, who is also currently working through her troubled relationship with oldest sister Kim Richards, was no longer speaking to Hilton as of a week before the party.
"Kathy's husband Rick is still mad at Mauricio [Umansky, Kyle's husband], for leaving [Rick's real estate company] Hilton & Hyland and starting [successful competitor] The Agency. The animosity has been building up for awhile. Mauricio isn't even welcome in the Hilton house," the insider told PEOPLE.
RELATED VIDEO: Kyle Richards Dishes on the Drama-Filled Season 6 of The Real Housewives of Beverly Hills
The sisterly stress has made its way into Richards' RHOBH, including details of how she was disinvited then re-invited to niece Nicky Hilton's lavish London wedding to banking heir James Rothschild this past July.
Richards did end up attending the wedding and posting about it on Instagram, including a sweet pic of daughter Portia, 7, who served as one of Hilton's flower girls.
Still, tension remained as not all of Richards' family was invited, including Mauricio and two more of her daughters.
And the strife goes both ways, said the source last week.
"Kathy is mad that Kyle brought Nicky into the fight," the source said, adding that neither Richards sister was invited to the Hiltons' annual Christmas party on Dec. 12. "There is so much drama in that family. It's sad that even over the holidays, they still can't get along."
