The reality TV stars' casting announcement was made on Wednesday's episode of Good Morning America.
Charli and Heidi will make history as the first-ever mother and daughter to compete on the reality dancing competition, which aired all 30 of its prior seasons on ABC.
"I was in shock," Heidi told GMA co-host Michael Strahan, recalling the moment when Dancing with the Stars asked her to be a contestant this Fall. "I was like, 'I can see [Charli] on it. [Why me]?"
And Charli admitted that she thought the whole thing was a joke or "a prank."
"When they were like, 'No, this is real,' I was so excited for the both of us, especially to do this together," Charlie shared on GMA.
Charli said her sister will definitely be voting for her, explaining with a smile, "She lives with me! She better be on my side."
But Heidi pointed out how she still cooks for her other daughter, which may give her an edge.
When asked if the girls are going to be competitive or encourage each other, Charli explained, "It would be nice to have some smack-talk, but she is still my mother. I'm 18 but she can still ground me if she really wants to, so I can't be too harsh."
Charli said she'll always remember that Heidi is her mother and respect her.
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Charli, who has a dance background and became widely known for her moves on TikTok, said she's looking forward to learning the technical aspect of ballroom dance.
As for Heidi, she admitted it's been a long time since she's received dance training.
"I was a competition dancer when I was in high school and I would dance on my high school dance team, but that's been a very long time. We'll see if it's like riding a bike and if I can pick up choreography," Heidi shared. "We'll see, but I'm excited."
Charli concluded that she'll be disappointed if her own father and sister don't vote for her.
"We're checking phones; I want to know!" Heidi joked.