When her grown-up baby brother, Boris (Morgan Spector), finally immigrates to America from his native Russia, Diana (Janeane Garofalo) can’t be happier. The mother of two teenagers, eighteen-year-old Alex (Raviv Ullman) and fourteen-year-old Mira (Sarah Steele), and wife to Misha (Daniel Oreskes), Diana kicks Mira to an air mattress on the living-room floor, giving her daughter’s upstairs bedroom to Boris, who, she says, “makes the girls pregnant only from looking. Always, he is like this.” It quickly turns out that Boris Fodorovsky is not quite the innocent, fresh-off-the-boat émigré he first appears to be, bringing mystery and danger to the family’s wacky Sheepshead Bay home. In fact, everyone has secrets they’re hiding in Erika Sheffer’s delightful off-Broadway debut, Russian Transport, which is having its world premiere at the Acorn Theatre at Theatre Row in a New Group production directed by Scott Elliott. Garofalo, in her first major dramatic theatrical role, does a fine job as the foul-mouthed matriarch, seamlessly going back and forth between English and Russian as she takes care of Mira, who is just discovering boys and wants to spend the summer in Europe; Alex, who is not exactly the mobile-phone salesman he claims to be; and big and burly Misha, who runs a car service from a small office connected to the house. Insults and jokes in two languages fly fast and furious on Derek McLane’s bilevel stage, but things get serious in a hurry when Alex and Mira discover some unsavory things about their uncle Boris. The acting is uniformly solid in this involving exploration of the immigrant experience, which unfolds in Russian Transport like the matryoshka dolls that Mira collects.