Audrina Patridge’s Ex Dating Heidi Montag’s Sister

 Chris Weeks/; Jason LaVeris/FilmMagic

Don't the Hills girls ever meet anyone new?

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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.

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

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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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