OK! Exclusive: Ashlee's Baby Bump Revealed!

While Ashlee Simpson and her fiancé Pete Wentz are doing everything they can to sidestep the truth that they're expecting their first child together this fall, recent pictures of the bride-to-be make it pretty clear that the little sis of Jessica Simpson has a little Fall Out Baby on board.

Photographed outside the Chart House Restaurant in Malibu, Calif., on April 12, where sources tell OK! that Ashlee did her best to power through a bout of morning sickness, the 23-year-old appears to already be showing the first signs of a baby bump! And with insiders saying that the first Wentz-Simpson offspring is due in October, that would mean the singer is nearing the end of her first trimester.

(photo: Gilbert Flores/Celebrity Photo)

Meanwhile, the Simpson family sources also tell OK! that Ashlee's baby news is not sitting well with her big sister. "Jessica never imagined that her tom-boy younger sister would have a baby first," the insider explains. "It would be very hard for her not to be jealous."

Friends say Jess has longed for a baby for years and has said on numerous occasions that she can't wait to be a mom. But following her very public divorce from ex Nick Lachey that dream was put on hold.

"Jessica couldn't be happier for her sister but still can't help feeling jealous," the source tells OK!. "She'd love to be planning a wedding with a baby on the way."

For all the details on Ashlee's pregnancy, including exclusive wedding information, pick up the new issue of OK! — on sale everywhere Thursday!