Kathy Hilton and Kim Richards reportedly snubbed their sister, Kyle Richards, on her birthday.

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Amid their Real Housewives of Beverly Hills feud, Kathy and Kim were said to have ruined Kyle's birthday by failing to celebrate the event alongside her and her family, including her daughter Portia, and her friends, Kris Jenner and Faye Resnick.

"Kyle celebrated her forty-seventh birthday at Craig's in West Hollywood Monday night. Noticeably absent from the gathering were her sisters, Kim Richards and Kathy Hilton," Radar Online reported on Tuesday.

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"Kim was invited to the dinner but Kathy wasn't," an insider explained. "Things continue to be very tense. Kathy didn't even call Kyle to wish her a happy birthday! Kyle knew that even if she invited Kathy, she wouldn't go. There continues to be bad blood because Kyle discussed Nicky's wedding on Real Housewives of Beverly Hills."

Plus, Kathy and Kyle's husbands, Rick Hilton and Mauricio Umansky, are still at odds over Umansky's decision to leave Hilton's real estate business and start his own company.

"Kathy won't be forgiving Kyle anytime soon," the source continued. "The wedding was the last straw in long simmering tensions. And Kathy's husband, Rick is still furious Kyle's husband Mauricio left the Hiltons' real estate company to go out on his own."

For more of the Hilton/Richards feud, tune into The Real Housewives of Beverly Hills season six, airing tonight and every Tuesday night at 9 p.m. on Bravo.