I don't think I'm giving anything away by saying that all three girls in the schmaltzy new movie from writer/director Garry Marshall (Pretty Woman, Beaches) could pass as "the other sister" of the film's title.

Oldest daughter Heather's lesbian instincts distress Tate-family matriarch Elizabeth (Diane Keaton), while hollow rich kid Caroline upsets father Radley (Tom Skerritt) because she would rather settle down than do anything with her hard-earned college degree. And then there's Carla (Juliette Lewis), the "slightly mentally challenged" one (in the words of the presskit), who was sent away as a child to a special no-visitors boarding school.

Carla, of course, is the film's namesake, presumably the daughter referred to only in euphemism because she is misunderstood by her family. When the film begins, mama and papa Tate have decided to take Carla back after years apart, only to be reminded of the difficulties of raising a retarded daughter.

Now that Carla is 24 years old, her parents can't really interfere with her independence, and Carla desperately wants to live as a normal person her age would. She enrolls in a technical school, pushes for her own apartment and falls for Danny (Giovanni Ribisi), a similarly special young man.

Lewis and Ribisi are marvelous in an otherwise questionable film. As the film's bright-eyed heroine, Lewis breaks free from her bad-girl persona and pours everything into her first truly sincere role. Ribisi compliments her well, expertly capturing his character's shy stutter and nervous finger movements. It's fascinating when you realize how much Ribisi conveys with just his hands.

At the opposite extreme, the usually dependable Keaton doesn't know what to make of her uneven character, whose virtually inexplicable mood swings are a byproduct of a one-dimensional part. Keaton's talents are wasted here, used only as an overbearing obstacle to the protagonist's goals. Too frequently, the film mistakes distracting details as character motivations, complicating roles like Tom Skerritt's likeable father with hints of an alcoholic past or similarly volatile elements.

Garry Marshall--the man responsible for such upbeat but paper-thin comedies as Laverne & Shirley, Happy Days and Mork and Mindy--approaches all of his material with the mentality of a 70s television producer. Like most of his projects, The Other Sister ricochets between a predictable story line and delightful little surprises (Danny uses his favorite scenes from The Graduate as a blueprint for his own romantic goals), weak character development and strong romantic appeal. What we finally get is a thoroughly inconsistent little romance, buried beneath a shameless slew of pop hits.

Critics have lambasted The Other Sister for its clunky melodrama and impolitic use of mental retardation. However, in their hurry to dismiss the film (for valid reasons), few critics have stepped back to admit that this is the type of movie that audiences will love.

Like Stepmom or just about any Robin Williams film, The Other Sister offers audiences a refreshing break from the complications of real life. In its attempt to spark laughs and jerk tears, the film makes light of the real issues. What dooms the film in my mind is not its fanciful approach, but Marshall's poor judgment in using mental retardation as his catch.

Marshall's audience-friendly version of mild retardation amounts to a cutesy combination of childish and hypersensitive traits. The Other Sister reduces to an uncomfortable twist on 80s films like Big and Vice Versa, with the charm of naïve kids posing as adults replaced with actual adults who behave as children.

It's a well-worn formula that usually pays off with audiences and the Academy, and the excellent performances by Lewis and Ribisi would almost be enough to save it here if it weren't for the sex issue. This may be a film with no foundation in reality, but Marshall errs when he plays the sexual attraction between his retarded characters for laughs.

A humorous scene featured in the film's previews shows Carla and Danny shopping for acceptable sexual positions in a copy of The Joy of Sex ("Page 84 is okay, but I don't want to do page 155"). The moment is simultaneously entertaining and uncomfortable, and it perfectly characterizes the way Marshall approaches sexuality in his film (it goes downhill from there).

Virtually everything else in the movie suggests that Carla is a mature young woman, yet Marshall occasionally chooses to exploit her childish intellect. Of all the story's possible messages, the one that shows through is that retarded people should be allowed to "do it" too (an issue handled more tactfully in the charming Belgian film The Eighth Day).

Sex is hardly the most important issue facing these characters, yet Marshall forces it to the front of our attention. Other episodes in the film make better use of Carla's condition. In one such instance, Marhall captures the complexity of a situation that is both humorous and painful when Carla agrees to get a makeover. At other times, as when we see the characters dressed in goofy Halloween costumes, he encourages us to laugh at their immaturity.

Carla and Danny don't need to be retarded for the story to work (the film's rich/poor subplot would have functioned just as well). Ultimately, The Other Sister is a film with too many characters, too much clutter and too little genuine substance. I would like to have seen more about the barriers these characters face, and less poking fun at their shortcomings. That would have made for a special movie.