The "Cheap Thrills" hitmaker recently faced controversy because of the casting of her frequent collaborator as an autistic girl (Music) who moves in with her newly-sober half-sibling Zu (Kate Hudson) - despite the fact the 18-year-old star isn't autistic herself.
And after passionately defending the casting on Twitter, Sia once again opened up about the controversy during an interview on Australia's The Sunday Project, explaining, "I realised it wasn't ableism. I mean, it is ableism I guess as well, but it's actually nepotism because I can't do a project without her. I don't want to. I wouldn't make art if it didn't include her."
Maddie - who has starred in many of Sia's music videos, including the promos for "Elastic Heart" and "Chandelier" - feared people would think she was "making fun" of autistic people.
"I bold-facedly said, 'I won't let that happen,' " Sia insisted, but added that she's come to realise she cannot "protect" Maddie from criticism.
"Last week, I realised I couldn't really protect her from that, which I thought I could. We sent it off to the Child Mind Institute and she received 100 per cent as performance accuracy. I realise that there are some things I can't protect her from as much as I try," Sia sighed.
The movie originally starred Shia LaBeouf who co-starred with Maddie Ziegler in Sia's "Elastic Heart" music video, but the "Honey Boy" actor was ditched by the singer following abuse allegations made by former girlfriend FKA twigs.