Jessica Simpson and Ashlee Simpson are teaming up for a new clothing line.

The singing sisters are to collaborate on a line of tween apparel featuring sportswear, denim and activewear in their first fashion venture together, which will be part of the Jessica Simpson Collection.

Jessica - whose fashion empire is set to be worth an estimated $1 billion this year - told WWD: "Creating a tween apparel collection was a natural extension for the brand. 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."?

Ashlee hopes her venture into fashion will be inspirational for young girls wanting to experiment with their personal style.

She said: "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."

Jessica is set to see her empire expand further this year, with new ready-to-wear and career separates lines also planned.

When she launched the collection, she sold just handbags, shoes and boots, but gradually expanded to include watches, clothing, cosmetics, luggage, fragrances and swimwear, which is all sold in major department stores in the US.