Jessica Simpson was reportedly furious after being mistaken for little sister Ashlee by lover John Mayer.
The 'Dukes of Hazzard' star - who recently went from a blonde to brunette - was with her new musician boyfriend when he allegedly pointed to a picture of Ashlee on a magazine cover and asked "How old is this picture of you?"
Jessica is said to have lost her temper with John and insisted she and Ashlee look completely different.
A witness told America's Star magazine: "Jessica read John the riot act. He definitely touched a nerve."
Though Jessica, 26, was once close to her 22-year-old sister, they have reportedly started to grow apart since Ashlee lost weight and dyed her hair blonde.