Maddie Ziegler leaving 'Dance Moms' for Sia film

Maddie Ziegler is saying goodbye to the Abby Lee Dance Company -- but all for good reason!

The 13-year-old dancer and her mother, Melissa Gisoni, will not be returning to "Dance Moms" after season six, a source tells ET. Her younger sister, 11-year-old Mackenzie, will also not be returning.

WATCH: Maddie Ziegler Stars In Sia's 'Big Girls Cry' Video

According to the source, Ziegler's mom announced the news to the rest of the cast on Wednesday during a taping of the hit Lifetime reality show.

But this won't be the last you'll be seeing of the petite Pittsburgh, Pennsylvania, native -- she's set to star in "Sister," a film project written and directed by her frequent collaborator, Sia.

Ziegler has already been featured in three of Sia's music videos, showing off her highly-impressive dance skills in "Chandelier," "Elastic Heart" and "Big Girls Cry." Now, the dancer will star in the singer-songwriter's first-ever film, which is based on a one-page story she penned eight years ago.

"I was too embarrassed to tell anyone I wanted to make a movie because I thought it would be seen as a vanity project because I was a singer," Sia, 40, told The Hollywood Reporter last November. "And then last year, after I made the 'Chandelier' video, I realized that I was pretty good at directing, so I felt a little bit braver."

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Last September, ET chatted with Ziegler, who couldn't help herself from gushing with excitement over moving on to "bigger and better things."

"I definitely want to be moving more into acting," she explained.  "Still keeping up my dancing, but also [do] movies and scripted TV shows."

She also told ET that when she met Sia for the first time, they became instant friends. "She's literally the sweetest person ever," Ziegler said. "I feel like I'm just so comfortable with her every time I'm with her. I never feel like, 'Oh my god, I'm hanging out with a celebrity.'"

"She's the most humble person and so caring, and she like, cooks for me!" she continued. "She's such an amazing person."

WATCH: Maddie Ziegler Raves about Sia: 'We Became Instant Best Friends'