"We've had a brilliant six years at Twenty8Twelve and are now looking forward to new pastures," the Miller sisters said in a statement. "We wish the brand the best of luck for the future."
It appears they're staying mum on why exactly they're leaving, though. Could it have something to do with Sienna's recently announced pregnancy?
The line will still continue, but starting with the fall collection, the label will read "Twenty8Twelve London" rather than "Twenty8Twelve by s. miller."
But it's been a good run for the line; the Miller sisters launched Twenty8Twelve in 2007 in conjunction with Pepe Jeans and Spanish entrepreneur Carlos Ortega. Savannah is a professional fashion designer, and Sienna became the face of the brand, modeling the line's collections over the years.
Don't fear, Twenty8Twelve fans: the brand's three stores in England will remain open, and they're currently in talks to open a New York location. Just know that (sadness!) you probably won't get a glimpse of Sienna while you're shopping.
Click over to WWD to read more about the history of Twenty8Twelve, and as a requiem, check out Sienna modeling some of the brand's clothing in the slideshow.