Writing great sister acts came naturally to Jane Austen. Or so we gather from Miss Austen, a poignant four-part Masterpiece drama adapted from Gill Hornby’s novel. Set in 1830, more than a decade after Jane’s untimely passing at 41, the story focuses on her surviving sister and dearest friend, the loyal and adoring Cassandra, who is determined to protect Jane’s privacy upon discovering a cache of her private letters written to her close friend, Eliza Fowle (Madeline Walker), also now deceased. (The series is unflinching in depicting the era’s shorter life spans.) Through these missives, the past comes vividly to life, spinning a fable of unrequited love and undying sibling devotion worthy of Jane’s own fictions. Cassandra’s younger self, called Cassy (a radiant Synnøve Karlsen), is described in Jane’s letters as “the calm at the ...