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Mauricio Umansky, Kyle Richards, Kathy Hilton, Rick Hilton

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When it comes to her burgeoning family feud with sister Kathy Hilton, Kyle Richards is finally putting her foot down. 

The 46-year-old Real Housewives of Beverly Hills star added a new level of drama to the ongoing off-screen saga between herself and her famous sister when she was offered an invitation to Hilton's Christmas party. A source told E! News Richards simply did not show up.  

"Kyle was invited but did not attend," a source exclusively shared with E! News. "She didn't want to go if Mauricio wasn't welcome."

In a previous episode of the popular Bravo reality show, fans learned Richard's husband Mauricio Umansky was not invited to Hilton's daughter, Nicky Hilton's wedding over the summer because of a mounting rift between Umansky and Hilton's husband Rick Hilton.

The separation has had a clearly negative effect on Richards' relationship with her older sister. 

"In the past, [Kyle]'s come for an hour or so by herself and then left, but this year it looks like she put her foot down," the source added. 

Kathy Hilton, Paris Hilton, Kyle Richards, Kim Richards

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It seems Hilton's sister was not the only relative to RSVP no to the holiday soirée. Her own daughters, Nicky and Paris Hilton, also never made it to the gathering.

"Neither Paris nor Nicky were able to make it, but they'll all be home in time for the actual holiday," the source said. "They'll be at Christmas dinner."  

We would just love to be a fly on the wall at that dinner.