“I’ve actually lived with [Sia] this year more than I have with my actual family, so it’s fun,” Ziegler, 15, told PEOPLE. “We have the best time together. She teaches me how to cook and new things about life.”
Ever since Ziegler began starring in the 41-year-old singer’s music videos — beginning with “Chandelier,” “Elastic Heart” and “Big Girls Cry” — the Dance Moms alum has become an inseparable part of Sia’s brand, often serving as the proxy face for the singer’s secretive identity in everything from her red carpet appearances, album covers and website. Sia spent this summer directing a film about her life, and she chose Ziegler to portray her as a child.
The duo spent this summer filming the movie, which is called Sister, and that led to spending a lot of time together.
“It doesn’t feel like work when we’re together anymore,” says Ziegler. “It’s really just a bond that we have. She’s my second mum.”
The cast of Sister will also feature Hamilton‘s Leslie Odom Jr., 36, and Kate Hudson, 38, who buzzed her hair for the part.
“It was such an incredible experience getting to work with Kate [Hudson] and Sia. I always work with Sia as a director, so that wasn’t something new, but her directing us for acting was [different and] incredible,” says Ziegler. “I had the best time.”
Ziegler previously told PEOPLE that Sia serves as her mentor, giving her advice on everything from dating to staying true to herself.
“I just want to keep inspiring girls, and I want to give off the right image. I don’t ever want to look too old or do something that isn’t for my age,” says Ziegler. “I just want to keep it young and fresh and inspire kids to be themselves. I hope I’m doing that, and I want to continue doing that.”
This article originally appeared on PEOPLE