A misbegotten dramedy about the hijinks of the mentally handicapped, Demi-sœur charts the adventures of developmentally disabled senior citizen Nénette (Josiane Balasko, who also directs) after the death of her mother compels her to track down her father — a search that instead leads to a reunion with heretofore unknown pharmacist brother Paul (Michel Blanc).
Nénette chats incessantly with her pet tortoise and befriends a heavy metal singer who gives her a bag of ecstasy he calls “sweeteners,” which Nénette then puts in uptight Paul’s coffee, turning him into a drugged-out loon who, the film posits in an especially sour joke, is now on her intellectual and emotional plane.
Out of his mind, Paul bonds with Nénette (by setting his pet hermit crabs free!) and reconciles with his estranged son, all while brother and sister tickle each other and prance about with a gaiety that’s as over-the-top as the rest of Balasko’s compendium of giggly and pouty facial expressions.
Shot with a high-def flatness that makes it look like a TV movie, the film indulges in much wannabe-funny wailing, shrieking, and flopping about by Nénette and Paul, only to then lace its buffoonish material with semi-serious undercurrents that eventually come to the fore in a crudely schmaltzy familial-unity finale fit only for the brainless.