Celebrity snoops at Star Magazine are dishing that Ashlee and Jessica Simpson have hit a sisterly bump in the road.

“There’s certainly a sibling rivalry between them,” a source close to the two tells Star. The trouble began, says another insider when Jess’s boyfriend, John Mayer, pointed to a glamorous magazine photo and asked Jess, “How old is this picture of you?” But-whoops!-it was a shot of Ashlee! “Jessica read John the riot act,” says our source. “He definitely touched a nerve.” Her sensitivity on the subject seems to be the direct result of Ashlee’s emergence as a sex kitten, a role once owned by Jessica. That Ashlee has become the hot blonde – a transformation that came courtesy of a surgeon’s knife, a bottle of peroxide and dramatic weight loss- is troubling to Jessica, who has morphed into a buxom and sometimes frumpy brunette due to a dye job and about 20 extra pounds. “Jessica hears the catty talk about how Ashlee looks like her- from two years ago,” says the source. “It totally bothers her.”