Sibling rivalry is bad enough for normal people, but these sexy and famous celebs don't just primp in the same bathroom, they have to share the spotlight.

PHOTOS: Bikini Wars! Sister Vs. Sister

The curvy Kardashian's, the scandalous Spears, and the sweet Simpsons are just a few of the sexy sister acts in Hollywood, who are constantly compared to each other when it comes to their careers, their looks, their personal lives and now, in RadarOnline.com's weekly Bikini Wars feature, their ability to rock a bikini!

Kelly Osbourne has taken to her Twitter page to attack Dannii Minogue, the popular Australian singer/actress who is a judge on Simon Cowell's X Factor in the UK. And it's all because of stories Dannii is telling about Kelly's mom, Sharon Osbourne, RadarOnline.com has learned.

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Dannii, younger sister of Australian pop superstar Kylie Minogue, has a new autobiography about to hit stands. In it, Dannii claims Sharon Osbourne mistreated her from their first day on the X Factor set together, berating her for not telling the press what a "nice person" she was. Dannii says the encounter left her in tears and ruined the entire season of the show for her.

Dancing with the Stars Alum Kelly, always the great protector of her family name, has gone Twitter-wild over the accusation.

"Dannii is the devil!" Kelly wrote.

And, "to the 12 Dannii fans out there I ask you this question, what would you do if someone wrote a book of lies about your mother?"

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X Factor, created and executive produced by Simon Cowell, is coming to US television Fall, 2011. Cowell moved his good friend Sharon onto the judges panel of America's Got Talent when he launched that show in 2007, and there is speculation he might move her again to his US X Factor bench next year.

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We're guessing he is not going to find a slot for Dannii.

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