The Anti-Jessica? Laid-back Ashlee Simpson Wants To ‘Enjoy’ A Stress-Free Wedding, Says Source — But With One Over-The-Top Surprise!

Ashlee Simpson famously sang about living life in big sister Jessica Simpson’s shadow. But when she walks down the aisle in just a few short weeks, she’s going to do it her way. RadarOnline.com has learned the laid-back bride-to-be is insisting on “enjoying” a “stress-free” planning process in the run-up to her big day.

“She actually doesn’t have wedding jitters,” a source told Radar. And it’s all because of her fashion designer sibling, who tied the knot with former NFL pro Eric Johnson July 5 at Montecito’s luxe San Ysidro Ranch during a lavish wedding weekend in front of 250 friends and family members.

“Seeing how much went into planning Jessica’s wedding, Ashlee wants to take the time to enjoy the wedding process and every minute before it comes,” added the source.

In fact, Pete Wentz’s ex, who’s slated to say “I do” with Diana Ross’s son, actor Evan Ross, 25, over Labor Day weekend at the legendary singer’s Connecticut estate, would rather leave all the details to everyone else.

“She loves that her mother is assisting with planning the wedding,” continued the source.

But, despite being care-free — and stress-free — about her nuptials, there is one must-have on Ashlee's checklist.

“All she wants is Diana to sing at the wedding,” said the source.

Simpson and Ross, who were spotted taking a break from wedding planning July 13 at her ex Ryan Cabrera’s 32nd birthday party at the Kia Motors Malibu Estate, announced their engagement during a Hawaiian getaway via Twitter in January after six months of dating: "My baby love and I are ENGAGED!!! Hallelujah Hawaii!!" Simpson wrote.