According to ET's source, Ziegler is leaving Dance Moms
to continue working with Sia. She's set to star in a film project written and directed by the 40-year-old pop star
, titled Sisters.
"She's literally the sweetest person ever," Ziegler told ET. "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.'"