Actress and model Sienna Miller, 25, has joined hands with designer-sister Savannah Miller to launch an 80 piece ready-to-wear fashion collection called Twenty8Twelve by s.miller for fall 2007.

Sienna and her sister were at London's Berkeley Hotel Wednesday to unveil the line, "I've always loved fashion, and Savannah got a First in design at Saint Martins,” she said. “We hadn't thought to pursue it together, but this just happened organically. And setting up a label with your sister is a lot of fun — there haven't been any fights yet.

According to WWD the first collection will include denim, knitwear, tailored jackets, dresses and outerwear.

The peculiar name for the collection is derived from Sienna's date of birth. She has deliberately kept her linkage to the collection subdued.

"I'm not about to become a brand," she explained. "Because of my job, people are willing to invest in the business, but my sister's an incredible designer, and it's a great opportunity for me to work with her."

Sienna and Savannah were equally involved in designing the collection backed by Spanish entrepreneur Carlos Ortega who owns brands such as Pepe Jean's for which Sienna currently models.