One sister gets married, then the other sister does. There's not much plot in Garry Marshall's new movie, but that's not what we came for. "The Other Sister" features a crackerjack performance by Juliette Lewis, the twenty-four- year old actress whose career has found her in vulnerable roles but with a determined sexual presence. In her most complex performance to date, Lewis plays Carla Tate, a mildly retarded young woman who seeks first a refuge from her overprotective mother, later opts for the independence of her own apartment, and finally goes for the same pleasures of domesticity that are the birthright of every decent person.

At the opening of the story, Carla (Juliette Lewis) is headed home from an institution for the mentally challenged, accompanied by her well-heeled dad, Bradley (Tom Skerritt). She has mixed feelings, and no wonder. Her mother, Elizabeth (Diane Keaton), does not treat her like the grown- up woman she is, but instead smothers her, insisting that she need not go back to school at least in the near future, and demonstrating virtually no trust in a rosy future for her slow-talking, slow-thinking girl. Apparently forgetting that Carla has at the same emotional needs as her other two daughters, Caroline and Heather, she is later dismayed to discover that Carla has formed an emotional bond with a mentally-challenged young baker, Daniel (Giovanni Ribisi) and wants an apartment of her own. While Elizabeth basks in the glow of Caroline, who is about to marry a devilishly handsome and articulate fellow, she is none too delighted with young Heather, a lesbian whose partner Michelle is ostracized and refused an invitation to Caroline's wedding.

The principal attraction of the movie is Juliette Lewis, whose portrayal of an intellectual dull and emotionally volatile 20-something is deeper and more affecting than was Dustin Hoffman's poker-faced performance as Barry Levinson's autistic "Rain Man." She draws laughs by her innocent way of fouling up dignified affairs. In one scene, her mother is being honored for her founding of a shelter for stray dogs. Carla blamelessly barks at one German Shepherd and opens the gates to free the animals from their benefactors. In the film's prize moments, Carla takes advantage of a free facial makeover at a San Francisco department store, not realizing that only half the face is to be a promotion, the other requiring fifty dollars. She ambles away from the store hiding her plain side from the staring crowds not unlike Brendan Gleeson's Cahill in John Boorman's "The General."

Giovanni Ribisi turns in a fine complementary role as Carla's intellectually limited boyfriend, Daniel, and in several scenes that are too cute by half, the two lovebirds negotiate a mutually satisfactory day to "do it." They follow the guidelines of Alex Comfort's "The Joy of Sex" as though it were a textbook for a course they are taking at a private academy--where Carla manages to get a passing grade in her computer science class.

Garry Marshall, best-known for directing "Pretty Woman," the remarkable "Frankie and Johnny" but also turkeys like "Dear God," evokes a stirring performance from Juliette Lewis while contenting himself with more one-sided dispatches from Diane Keaton as smother-mother and Tom Skerritt as a caring, but bland and underdeveloped dad. Life among the wealthy in a San Francisco suburb is sharply photographed by Dante Spinotti in a film which at first sounds like yet another insipid "Stepmom" but emerges as a winning account of the fears and joys of a remarkable young woman.