The Other Sister casts Juliette Lewis and Giovanni Ribisi as two mentally challenged kids in love. It's not the emphasis on tics and grimaces that mars their essentially well-meaning performances, it's the sitcom crassness of director and co-writer Garry Marshall. Girl to boy, as they study a sex manual: "Who invented sex?" Boy: "I think it was Madonna." Is it any wonder that good actors, such as Diane Keaton as Lewis' mother, go down with this tear-jerking ship?