Singer-songwriter Lizzy Pattinson chose not to use her brother's famous name when she sang for the panel during auditions earlier this summer - but Simon wishes she had!
Speaking on ITV's This Morning, head judge Simon said: "I don't like to know anything [before someone auditions]… but there was a girl that came in who turned out to be Robert Pattinson's sister.
"They [production] didn't tell us, and in fact she didn't tell us until afterwards, which was a bit annoying.
"But I don't like to know. [It's more fair that way]."
According to sources, Lizzy, 31, was determined that the panel didn't know her famous relative's identity as she wanted to succeed on her own merits.
"She didn't mention she was related to him to any judges or producers," a source told The Mirror. "It's important to her she does not fall into the trap of just becoming Rob's sister."
Now that the secret is out, however, proud brother Rob can't help gushing about his sister's talents!
"I'm so ridiculously proud of her," he's been quoted as saying. "I would never have the balls to do it in a million years. I can't believe she's done it. It's amazing."
X Factor returns for its 11th series on Saturday, 30 August on ITV1.
Watch the official X Factor 2014 trailer below:
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