Everyone yearns for their mom when they're sick, and Liza Minnelli's 1948 film for comfort after she had to cancel a March 30 Las Vegas concert with Michael Feinstein due to a viral infection.

"Liza was more upset about backing out of the show than she was about being sick," an insider tells Closer. Her love for the stage is so strong that she's fighting to get back to it as soon as she can. "She's the Energizer bunny," brother Joey Luft, 63, assures Closer of Liza's resilience.

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"She's had every imaginable surgery — both knees, hips, back. She's a trouper, and I'm sure she'll battle this and be OK," he added. Joey, the son of Judy and Sid Luft, says he's checking up on Liza regularly while also touching base with his other sister, Lorna Luft, 65.

"She collapsed after doing a show in London and they found a brain tumor," he reports of a March 9 incident. "She had surgery [back in the US] and they got the tumor out, so I'm trying to visit her in Palm Springs," Joey says of Lorna, who's in remission from breast cancer. "She's really a survivor — just like Liza is!"

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Liza with Joey and Lorna.

And though the insider reports Liza's still fighting off the virus and it could be "a couple months" before she's well enough to reschedule her Vegas concert, her spirit is determined. "This is what I love most, singing with my guy, Michael," Liza says. "I've still got it," she adds, making a promise to her fans with a nod to one of her mom's tunes. "I've still got a lot of livin' to do!"

For more on Liza Minnelli, pick up the latest issue of Closer Weekly, on newsstands now — and be sure to sign up for our newsletter for more exclusive news!