The pair will be playing mother and daughter in Nikole Beckwith's feature directorial debut, reports Deadline.
Stockholm, Pennsylvania follows Leia (Ronan), a developmentally stunted young woman who was raised by her kidnapper.
After 20 years of separation, Leia is reunited with actual parents (Nixon and David Warshofsky) but struggles to adapt to her new life.
Harry Potter and Awake star Jason Isaacs will play Leia's captor.
Ronan can be next seen in Wes Anderson's upcoming film The Grand Budapest Hotel, which releases on March 7 in the US and UK.
Nixon, known for her character Miranda in HBO's Sex and the City, has appeared in several television series including The Big C and Alpha House.