Josiane Balasko's Demi-Soeur suggests that modern pharmaceuticals can abet the storytelling in an old-fashioned sentimental farce: A dose of Ecstasy is all that's required to activate the relationship between Nenette (Balasko), a 60-year-old with the understanding of a first-grader, and her previously unknown half-brother Paul (Michel Blanc).

But if the "love drug" can heal characters like Paul, a crabby provincial pharmacist, it's not as curative for a limping screenplay like the one co-written by Balasko and Franck le Joseph. Sweet but silly, Demi-Soeur leaves plot strands flapping as it romps to a hasty conclusion.

The movie opens with fluffy clouds and Christophe Julien's Michel Legrand-like score, announcing that the story is set in a Gallic wonderland — even if the first stop is Nenette's mother's funeral. "Blessed are the simple hearts," intones the priest, as the bereft woman cuddles her only friend, a tortoise named Tootie.

Without mom to protect her, Nenette must move to an institution, The Lindens. Because she doesn't want to go, her temporary caregivers distract the gray-haired child with a box of old photos. That's where she finds a letter from the father she never knew, with a return address in Angers.

Nenette arrives at The Lindens, and learns it has at least one disagreeable rule: no pets. So she and Tootie sneak out and hit the road, on foot, to Angers. Halfway there, Nenette spots a rabbit and follows it to a different sort of wonderland: an outdoor rave. There she's befriended by a shaggy-mustached roadie (George Aguilar, Balasko's husband) and an all-female hardcore punk band, the Black Iron Bitches. Later, when the cops arrive, Nenette is hurriedly handed the musicians' pills.

Those will come in handy, if only by accident, after Nenette learns that her father is dead, but that she has a brother. Although the demi-siblings do have something in common — she has that tortoise, he collects snails and hermit crabs — Paul is not happy to see her when she turns up on his doorstep.

After Nenette innocently pops MDMA in his coffee, though, Paul warms to his half-sister considerably. He introduces her to his son and granddaughter — who is, mentally, about her great-aunt's age — and to some other relatives. The latter are suspicious, but they and their concerns are soon forgotten; an intricate plot in which Nenette's arrival somehow fixes the entire family's problems seems to have been the idea, but both Demi-Soeur and its protagonist have short attention spans.

Grumpy, miserly Paul's secret vice turns out to be Mozart, which is characteristic of this amiably retro comedy. Balasko clearly didn't do much research into youth or drug subcultures, and doesn't worry about things like whether or not a punk band would play a rave. The detours into kid stuff, meanwhile, are just meant to spin the film back onto the road toward cozy (if sometimes slightly vulgar) French domesticity.

Balasko is probably best known in the U.S. for 1995's French Twist, another self-directed vehicle. It was a sex (and sexual-orientation) farce, but that sort of humor is relegated to the margins in Demi-Soeur. Then again, too many condom and mistress jokes might seem inappropriate in a film about a grown-up who's never experienced puberty, at least not mentally.

If Nenette as a character is more a narrative convenience than a depiction of an actual condition, her permanent childhood does provide the 63-year-old Balasko with an exuberant, unpredictable role. That she continues to make work for herself as both an actress and a director is a good thing, but it would be better if she found a more ambitious writer.

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