EXCLUSIVE: Briana Cuoco Receives Best Advice From Sister Kaley: Don't Be a Jerk, Work Hard

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When two sisters are both in the business, they gotta stick together!

Singer Briana Cuoco, who is starring in the upcoming Lizzie the Musical as real-life axe murderer, Lizzie Borden, revealed to ET she has benefited from some sound advice given to her by big sister -- and Big Bang Theory star -- Kaley Cuoco.

"Don't be a jerk. Keep your feet on the ground," Briana shared. "Work hard, and don't take yourself too seriously."

WATCH: Kaley Cuoco is Giving Us Some Serious Body-Envy During Her 'Lip Sync Battle' Performance!

That's pretty good advice!

Briana, 27, who has appeared as a contestant on NBC's The Voice, and also guest starred on Big Bang, explained that her loyalty to Kaley runs deep.

"People can be especially mean on Instagram," she said, adding that when she sees nasty comments about her sister online, "I shut them down!"

WATCH: Josh Gad Makes Out With Johnny Galecki as Kaley Cuoco Watches

"My sister fangs come out," Briana noted. "She actually texts me and says, 'Sis, thanks for having my back.'"

As for how she landed her role playing an infamous murderer, Briana joked that, sadly, it made perfect sense to people.

"I've just met so many people over the years that look at me and they're like, 'Axe murderess, I see it!'" she said, laughing.

WATCH: Kaley Cuoco Proudly Shows Off Her Amazing 'Bachelor' Ben Higgins Yoga Pants

"It's heavy rock music, which I love singing personally," the vocalist added of the show, which is being staged at the Mint in Los Angeles on March 3-4. "She's so fascinating, she obviously has a little bit of screws loose up there, which is so fun to play. I'm so excited."

Meanwhile, Kaley gave us serious body envy during her recent performance on Lip Sync Battle.

Watch the video below!