JULIA ROBERTS is staying away from her sister’s wedding, a source says, because she hates the groom and doesn’t want to get stuck with the bill!

Nancy Motes – the star’s younger half sister – asked her famous sibling to be a brides­maid in her upcoming nuptials to John Dilbeck.

Julia not only said “No” – she’s even refusing to go as a guest, the source added.

“Julia doesn’t think much of Nancy’s husband-to-be,” revealed an insider. “She’s convinced he’s sponging off Nancy, who doesn’t have much money in the first place.

“They moved out to L.A. from Georgia, and John was working at a Panda Express restaurant. But he quit his job after Nancy landed a gig as a production assistant on (the TV musical) ‘Glee.’”

John recently made a splash by getting down on bended knee and proposing to Nancy in front of cast and crew members on the Hollywood set of the hit FOX show.

Ironically, the source says it was Julia who got Nancy the job by pulling strings with show creator Ryan Murphy, who directed the 45-year-old actress in the film “Eat Pray Love.”

This is just the latest in a long list of battles between the sisters.

In 2005, Julia exploded when she caught Nancy, 37, snap­ping pictures of her then-newborn twins, Phinnaeus and Hazel, at the actress’ home in Taos, N.M. And in 2011, the two got into a heated fight after Julia made dis­paraging remarks about Nancy’s weight.

“Now, Julia won’t go to her wedding because she’s worried Nancy will try to tap her for more money,” said the insider.

Apparently, the sisters’ mom, Betty Motes, is try­ing to play peacemaker and patch up the feud before Nancy walks down the aisle later this year.

But the insider said: “Julia told her not to hold her breath.”