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‘Lip Sync Battle’ Pits Julianne Hough Against Derek Hough for a Knock-Down, Drag-Out Sibling War (Video)

”Dancing With the Stars“ champions square off on the Spike show

(Spoiler alert: Please do not read on if you haven’t watched Thursday’s episode of “Lip Sync Battle.”)

“Dancing With the Stars” champions Julianne and Derek Hough took their sibling rivalry to the next level in Thursday’s “Lip Sync Battle” on Spike.

Two-time “DWTS” champion Julianne once again faced off against her five-time champion brother — but this time in the lip-sync realm.

“This is going to get ugly,” warned host LL Cool J, as Derek stepped forward. “This is your chance for a little sibling revenge.”

Derek started out with a song he claimed his mother used to sing to him, “Can’t Hold Us” by Macklemore & Ryan Lewis (ft. Ray Dalton). Giving an energetic performance in loose jeans and an open peacoat jacket over a white T-shirt, the dancer later proclaimed his rendition “perfect.”

“No, I’m just kidding,” he laughed.

His sister, an actress and “DWTS” judge, claimed he cheated, employing gyrations to distract from the battle’s primary focus: “I feel like you pulled out all your stops with your pelvic thrusts — all of that stuff. That’s your move. That’s not even fair. This is a lip-syncing contest.”

Dismissed by LL Cool J, Derek traded places with his sister, who took center stage for Meghan Trainor’s “All About That Bass.” Julianne pulled no punches and proceeded to put her brother’s relatively modest opening to shame, doing sassy hair flips and wide hip swings in a snug, flame-red midriff-baring top-and-skirt combo. She was then joined onstage by Trainor herself, who ran her hand over Julianne’s bass to great audience applause.

“What’d I think?” Derek said. “It’s wildly inappropriate. And — I just call for a disqualification. You can’t do the real thing! Meghan, I thought we were friends.”

When the Hough vs. Hough family grudge match continued, Derek donned a white lob wig and performed Sia’s “Chandelier” with four background dancers in ethereal, wispy dance costumes. Cohost Chrissy Teigen swayed, holding up a lighter for the interpretive-dance(ish) performance.

“You’re about to pull the bazooka out,” LL Cool J said to Julianne in an aside that promised/forewarned that Julianne was about to lay the real smackdown on her bro.

In a semi-confessional later, Julianne admitted that she intended to destroy Derek, who, she said, resents her judging him on “DWTS”: “Winning this competition is probably the most important thing — in history — because the rivalry between Derek and I. I judge him on ‘Dancing With the Stars.’ He says it’s because, ‘Those who can’t do, judge.’ … He’s so going down.”

Sure enough, she returned with guns blazing, performing the Lonely Island’s “I Just Had Sex” with two shirtless male dancers.

Her brother’s mortified expression said it all: Julianne won the night.

Derek: “I’m straight-up traumatized right now.”

See a video of Julianne’s “I Just Had Sex” lip sync here, and Derek’s performance of “Can’t Hold Us” here.