The main character of The Other Sister is Carla Tate (Juliette Lewis), a mentally retarded teen who has just returned home from an institution. Her rich parents, Elizabeth (Diane Keaton) and Radley (Tom Skerritt), don't quite know what to do with her. Carla's goal is to get a high school diploma and become a veternarian's assistant. Elizabeth thinks that's out of the question.
But Carla is determined to declare her independence, whether her parents approve or not. Soon, she's enrolled in the local PolyTechnic school, and hunting for an apartment of her own.
The final test of her parents' patience comes when she meets Danny (Giovanni Ribisi), a classmate of hers who is also mentally retarded. The two form a quick friendship, and fall in love. But when the topics of sex and marriage are raised, Carla's parents are at a complete loss.
Juliette Lewis and Giovanni Ribisi apparently attended the same acting class on the subject of portraying the mentally handicapped. They both deliver the same affectations, and an overabundance of mannerisms that bely a true lack of confidence in their characterizations. They are never able to get lost in the characters; in each and every scene, you are always acutely aware that you're watching someone acting.
The script provides them plenty of crutches to work with: She has a childlike affinity for animals, and he has a childlike affinity for marching bands. She throws a childlike tantrum when people laugh at her, he throws a childlike tantrum when he is called "stupid". But these lovable quirks are the end-all and be-all of their characters. And it doesn't end with Carla and Danny...every single character in the film can be singularly identified by a defining quirk.
The Other Sister has its cute moments, several of which actually do work. However, the majority of the movie is elaborately over-staged. The film is perfectly willing to sacrifice believablility for a quick laugh.
The movie is also surprisingly long. It clocks in at over two hours, an excessive amount of time for what is in actuality a simple romantic comedy.
The Other Sister is much too self-conscious to pack any dramatic weight. But, as a lightweight, throwaway romantic comedy it's harmlessly entertaining.