"Of course," the 27-year-old told PEOPLE when asked if she'll be standing next to Ashlee on her big day.
Performing in front of nearly 35,000 people at Camp Pendleton Thursday night, Jessica was in fine form for the troops, belting out "These Boots Are Made For Walkin' " at a three-hour concert that also featured Snoop Dogg, Janet Jackson, Ne-Yo, and Pamela Anderson. Taking the stage in a skin-tight red top, jeans and black stiletto heels, Jessica was greeted by howls and hoorahs.
Little sister Ashlee, 23, and fiancé Pete Wentz (of Fall Out Boy fame) have kept mum on their wedding plans – and her pregnancy.
But she can't help gushing about her engagement.
While promoting her new album, Bittersweet World, in London, Ashlee proudly showed off her ring on a British chat show saying, "He did a great job." As for her husband-to-be? "I got a good one."