Audrina Patridge’s Ex Dating Heidi Montag’s Sister

 Chris Weeks/WireImage.com; Jason LaVeris/FilmMagic

Don't the Hills girls ever meet anyone new?

Holly Montag, 27, is the latest alum of the MTV hit to give romance a chance with former costar Justin "Bobby" Brescia, 29 — the same commitmentphobe who once courted Audrina Patridge, 25, and Kristin Cavallari, 24.

"They're dating and have been for awhile," a source tells Us Weekly (on stands now).

But another insider doesn't have as much faith in the relationship between Brescia and Heidi Montag's estranged sister. "He's reckless and will disappear in his house for days at a time," the source says. "He's not reliable."

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