Jessica and Ashlee Simpson's mother came up with the idea for their clothing line.
The girls' mother, Tina, was the first person to suggest they combine their two individual styles in a clothing line aimed at young girls.
Jessica told Fashionista.com: "My mother thought it would be great to combine both of our styles, both of our personalities."
She added that while it is the first time they have collaborated professionally, "It feels like we always work together."
Describing the differences in their style, she calls her look "pretty classic, I'm pretty easy going," while Ashlee has more of a "rock 'n' roll, cool, boho-chic," look, adding: "Combining the two is always fun."
Despite being sisters Ashlee and Jessica never ended up squabbling during the creative process either.
Jessica added: "We loved being in the approval meetings together, we loved inspiration shopping together.
"It's been fun. I feel like all these little girls are wearing stuff that Ashlee and I would both wear."
The sisters launched their range at Dylan's Candy Bar last week, when their T-shirts, shoes and jeans were launched.
Jessica - who also has a self titled signature line - is set to be busy as she has also signed a deal to design a maternity line, girls' footwear and her spring 2013 women's collection.