Field will play the mom on the show, which slides into "Grey's Anatomy" 10 p.m. Sunday timeslot, right behind "Desperate Housewives," Daily Variety reported Tuesday.
It marks Fields' first regular primetime role since 2002 when she was in the ABC drama "The Court."
Matthew Rhys will also be joining the cast, taking on the role handled by Jonathan LaPaglia in the pilot.
The Touchstone-produced "Brothers & Sisters" revolves around five siblings in California who are bound together by family and financial ties.
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