JESSICA SIMPSON and her sister Ashlee are locked in a wedding showdown!

Big sis Jess – who’s plan­ning to tie the knot with fiance Eric Johnson in a splashy affair this spring – is furious that Ashlee is racing to the altar with boyfriend Evan Ross after only a few months of dating. And she’s convinced that Ashlee is more interested in playing a game of one-upmanship than actually getting hitched!

The wedding war erupted after Evan, the 25-year-old son of Motown legend Diana Ross, told a reporter: “I’m so in love (with Ash­lee) and we’ve got amazing things go­ing on. Yeah, she’s the one.”

When asked if marriage was in their future, Evan replied: “For sure.”

Meanwhile, Jessica, 33, and Eric, 34 – par­ents to 19-month-old daughter Maxwell and 5-month-old son Ace – traveled to Italy in October to scout venues for their planned nuptials.

“As soon as Jessica tipped her family off that she and Eric finally carved out some time to marry this spring, Ashlee started planning her own wedding,” said a source. “Ashlee wants to stage a winter wonderland-themed extravaganza at her Los Angeles home early in the year.”

Jessica doesn’t think the timing is a coincidence.

“Ashlee has al­ways played follow the leader with Jes­sica,” the source explained. “When Jessica went on reality TV, Ashlee followed. Ashlee even dyed her hair blonde and softened her style from punk to more feminine to look more like Jessica.”

While Ashlee, 29, may win the race down the aisle, she stands to lose a whole lot more.

“Jessica feels betrayed,” the source added. “She’s not going to forget it.”