Credit: Jamie McCarthy/

They've conquered the music industry, and now Jessica and Ashlee Simpson are hoping to take over the world of fashion, too.

The Southern sisters will design a line of tween apparel as part of the Jessica Simpson Collection, WWD reports.

"Creating a tween apparel collection was a natural extension for the brand," Jessica, 30, says. "I'm excited to bring my sister, Ashlee, along as co-creative director of this division, because she brings a savvy rocker edge that defines today's tweens."

The line will hit stores this fall and will include sportswear, denim and activewear reminiscent of the sisters' "girly, playful attitudes" toward fashion. The collection will be comprised of knit tops, hoodies, pullovers, graphic T-shirts, striped shrunken knits, drawstring pants, footwear and outerwear.

"My goal as co-creative director will be to design clothes that will help inspire young girls to express their personalities and creativity through fashion," Ashlee, 26, says.

Jessica's fashion empire is estimated to make $1 billion in sales this year.