Remember the good old days when Jessica and Ashlee Simpson were best friends? Like the hair colors they’re now sporting and the guys they are seeing, the relationship between the once-close siblings is no longer what it used to be, according to Star magazine.

While their reps insist the sisters are still close, a tsunami seems to be building. “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.”

-Ashlee Simpson Pictures
-Jessica Simpson Pictures