Pregnant Jessica Simpson Wears Colorful Bridesmaid Dress at Wedding

Bridesmaids, starring the Simpson sisters!

Jessica and Ashlee Simpson were bridesmaids at the Saturday wedding of Lauren Zelman, their friend and former assistant, to TV actor Bret Harrison ("Breaking In," "V," "Reaper").

PHOTOS: Jessica's bump watch

And though Jessica, 31, is in the very late stages of her pregnancy, she fulfilled her bridesmaid duties for the weekend at the Parker Palm Springs alongside fiance Eric Johnson; Ashlee, 27; her beau, Vincent Piazza; her son Bronx; and their parents, Joe and Tina Simpson.

"She was a good sport about being a bridesmaid even though she is so pregnant," a source told Us Weekly. "It definitely wasn't easy for her to get around and help the bride."

PHOTOS: Her pre-pregnancy fashion flubs

For the ceremony, the "Fashion Star" mentor wore a pink bridesmaid dress (one matching the other women in the party) and was the last to arrive for the outdoor ceremony. With a little bit of help from a wedding coordinator, the singer walked down the aisle "very slowly" to avoid falling, an onlooker explains.

(Her adorable nephew Bronx, though at one point teary-eyed, was a crowd pleaser at the bash as a ring-bearer. Groom Harrison's onetime costar on "The OC," actor Adam Brody, was also in the wedding party.)

PHOTOS: Jessica at her first wedding to Nick Lachey

Once the couple said their "I dos," Simpson and her own husband-to-be linked arms as he helped her back up the aisle.

The extended Simpson clan took advantage of the fun weekend for some pool time and good eats at the restaurant Norma's, where Simpson noshed on an omelet.

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