Model-turned-actress Sarah Bradford, 22, grew up with the Oscar winner, after her mother and his father married.
She insists she's never used Damon to help her progress in Hollywood, and earned an audition for a role in The Adjustment Bureau through her own hard work.
But she was shocked when casting bosses told her she would have to jump into bed with her sibling.
Bradford tells New York magazine, "Five months ago I went in for a reading for this movie Matt's in, The Adjustment Bureau. It's in production right now in New York.
"It was for a character called Elise - she is basically queen bee at this ballet academy - and all of a sudden they say, 'Are you aware that if you get this role, you'll be playing Matt's love, having a love affair with your brother.'
"My jaw dropped."
Fortunately the sordid story ended on a positive note for the young actress: "I just read for a more appropriate role in the movie, Elise's friend, and I got the part. It's very exciting. What parents wouldn't want their children to work together."
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