Bobby Ann Mason wrote "Shiloh" in 1982. It originally appeared in The New Yorker magazine.
Norma Jean and Leroy are having problems in their marriage because of an injury that keeps him from working. While they are trying to work out their problems, Norma Jean's mother Mabel (Mama) comes to visit:
When [Mabel] visits, she inspects the closets and then the plants, informing Norma Jean when a plant is droopy or yellow... For the past few years, she has been urging Leroy and Norma Jean to visit the Civil War battleground [in Shiloh]. Mabel went there on her honeymoon -- the only real trip she ever took.
(Mason, "Shiloh," Google Books)
Her presence makes Norma Jean uneasy since she can't seem to stop interfering with their lives. Eventually, she catches Norma Jean smoking, which starts the events that lead to the finale; Norma Jean feels that with Leroy home all the time and Mabel coming by to criticize, she is trapped in her house like a teenager again, without freedom. Mabel serves to accentuate the irritation that Norma Jean feels with Leroy, since she is at first unwilling to take Leroy directly to task for his laziness. Mabel's suggestion that they visit Shiloh shows Norma Jean that her marriage is simply not working out, and that she needs to take her life into her own hands if she wants happiness.