EXCLUSIVE: Margo Martindale has joined the cast of August: Osage County. The Emmy winner will play the jaded sister of Meryl Streep in the feature adaptation of Tracy Letts’ family clan play. Martindale’s character is Mattie Fae Aiken, the wife of Charles Aiken, played by Chris Cooper, and the sister of Violet Weston, played by Streep. The actress will start filming in Oklahoma on September 24. Martindale won a Primetime Emmy for Outstanding Supporting Actress in 2011 for her role on FX’s Justified. Julia Roberts also stars in August: Osage County as Violet’s daughter Barbara. Abigail Breslin, playing Barbara’s daughter, recently joined the film, as did Juliette Lewis. John Wells is directing August: Osage County, Letts is writing the adaptation of his Pulitzer and Tony-winning play. George Clooney and Grant Heslov are producing along with Jean Doumanian, Steve Traxler and Claire Rudnick Polstein. The Weinstein Co. is financing and distributing August: Osage County. Martindale is repped by the Gersh Agency.