Margo Martindale has joined the cast of "August: Osage County," a spokeswoman at the Gersh Agency told TheWrap.
The actress will play the sister of Meryl Streep's character Violet Weston in the feature adaptation of the Pulitzer Prize-winning play, which premiered in Chicago in June 2007.
In the dark-comedy clan story, Martindale stars as the incessantly blasé Mattie Fae Aiken, wife of Chris Cooper's character Charles Aiken, who antagonizes her child and husband and eventually reveals that another man fathered her son.
Julia Roberts will star as Martindale's daughter, Barbara.
The Weinstein Company-financed film will begin shooting Sept. 24 in Oklahoma.
Martindale won a Primetime Emmy Award for Outstanding Supporting Actress last year for her performance at Mags Bennett in FX's "Justified."
Former "E.R." producer John Wells was tapped to direct "August," and playwright Tracy Letts -- who penned the original Pulitzer and Tony Award-winning play -- is writing the adaptation, which will be produced by George Clooney and Grant Heslov.
Deadline first reported the news.