Donald Margulies’s new play, Long Lost, almost revels in overly familiar plot elements. Focusing on two brothers who haven’t seen each other in years, Margulies draws on the good brother/bad brother dynamic of the Cain and Abel story; it pops up in Hollywood films as different as Arsenic and Old Lace and Legends of the Fall, but perhaps most pertinently in Duel in the Sun, where the brothers form two points of a love triangle. Here the siblings are David (Kelly AuCoin), a successful consultant, and his older brother, Billy (a gray-bearded Lee Tergesen). In Margulies’s story. David’s wife Molly (Annie Parisse) glancingly forms the third point. But another oft-mined trope is also at play: the stranger who arrives in a settled household and disrupts it is a staple of drama from The Playboy of the Western World to Picnic.