Jessica Simpson is going to be her sister Ashlee’s maid of honour.

The singer-and-actress - who is dating American football star Tony Romo - will be standing next to 23-year-old Ashlee when she ties the knot with Fall Out Boy rocker Pete Wentz.

When asked if she would be the maid of honour at the nuptials, Jessica replied: “Of course!”

The ‘Dukes of Hazzard’ star strutted her stuff on stage in front of 35,000 members of the Marine Corps at Camp Pendleton on Thursday night (08.05.08), performing her hit track ‘These Boots Are Made For Walkin” while wearing jeans, a tight, red top and black heels.

Janet Jackson, Snoop Dogg and Ne-Yo also performed at the concert.

Ashlee previously admitted 28-year-old Pete did “a good job” proposing, and believes she has “got a good one”, referring to his future husband potential.

Ashlee - who is rumoured to be pregnant - and Pete have been dating for nearly two years.

When she’s not putting on the ritz for work, Jessica Simpson leaves the makeup and fashion of her diva days behind. Life & Style reports it’s her romance with Tony Romo that deserves the credit for Jessica’s new laid-back look.

“Gone are the days of Jessica having her hair and makeup people on call 24/7,” a close source told the magazine. “She always worried about how she looked. Now she knows she can dress the way she wants and choose not to wear makeup and (Tony will) still love her just as much.”

In fact, according to the insider, Tony prefers it. “He’s more attracted to her when she’s casually dressed with her hair hanging around her shoulders, than when she’s carefully coiffed and dressed to the nines.”

Jess’ diva-downed style is a close copy of Tony’s own casual attire, featuring T-shirts, sweats and baseball caps. But lest anyone thinks this is a case of Jessica changing for a man, the source assures Jessica just feels comfortable enough with Tony to be herself. “She hates trying to look sexy all the time. Jess is a down-home girl on the inside.”

That’s not to say she’s totally turned her back on her famous fashion sense. “She still loves a great pair of shoes,” another insider shared.

Out in a Dallas Cowboys sweatshirt, Jessica Simpson was spotted toting around her pet pooch Daisy while running errands in Nashville, Tennessee on Friday (April 18).

The Dukes of Hazzard actress is doing her best to return to proper health after her recent hospitalization for a kidney affection - with reports claiming that the troubles resulted from her heavy drinking.

A source alleges that the affection was brought on by drinking an “obscene amount of alcohol” soon after she started dating Cowboys quarterback Tony Romo.

“She was a nervous wreck. She was three weeks late and convinced she was pregnant. She was feeling really weak and really scared. She was unraveling,” the insider says.

In the days since news of Ashlee Simpson’s possible pregnancy broke, Simpson and her fiancé, Pete Wentz, have issued responses that have sometimes sounded like denials and sometimes just seem cagey. Since their reps have yet to issue a firm “no” on the matter, many outlets are standing by their sources and their story, that the 23-year-old pop star is indeed expecting.

OK! is one of those outlets, and the magazine reports that older sister Jessica is happy for Ashlee, but is having a tough time with the news. “Jessica Simpson has yearned for a baby for years. She’s told friends that she can’t wait to be a mom and has even joked that if she has to, she’ll resort to making her hairstylist pal Ken Paves the adoptive daddy,” the magazine reports.

“Jessica never imagined that her tomboy younger sister would have a baby first,” a source close to Jess told OK! “It would be very hard for her not to be jealous.”

A source close to Ashley concurs: “Jessica couldn’t be happier for her sister but still can’t help feeling jealous. She’d love to be planning a wedding with a baby on the way.”