Even the title is ill-conceived in this tone-deaf movie about an amiable doofus who intrudes on the lives of his three bitchy sisters and redeems them through his hapless naivete. “Our Idiot Brother” aims to combine hard-nosed observational comedy with genial charm before opting for a warm, fuzzy denouement that will send everybody off feeling good. But Jesse Peretz and writers Evgenia Peretz and David Schisgall don’t find the balance that would make the unstable mixture palatable.

Paul Rudd evinces a pleasant, shambling charm as Ned Rochlin, a shaggy-haired goofball living with girlfriend Janet (Kathryn Hahn) on an organic farm whose produce they sell in an open-air stall at the farmers’ market. The guy’s so eager to please that when a uniformed cop approaches him with a sob story and asks if he can direct him to some weed for personal use, Ned happily sells him some, earning a stint in prison for his trouble.

Paroled early for good behavior and finding that Janet has taken up with dim-bulb Billy (T.J. Miller), Ned initially crashes with his supportive mom (Shirley Knight), but soon decides to check in with his three sisters, managing to ruin things for each of them seriatim. First up is Liz (Emily Mortimer), a put-upon wife whose husband Dylan (Steve Coogan) is a documentary filmmaker having an affair with his latest subject, a ballerina named Tatiana (Lydia Haug). Ned, reluctantly taken on by Dylan as his assistant, accidentally spills the beans, sabotaging the marriage. He also manages to upset the couple’s plans for their son River (Matthew Mindler), a shy kid who takes to his uncle’s rambunctious ways overmuch.

Next comes hard-driven Miranda (Elizabeth Banks), a Vanity Fair writer whose exclusive with a Fergie-like celebrity aristocrat (Janet Montgomery) Ned botches, while also complicating her friendship with BFF Jeremy (Adam Scott).

Last is Natalie (Zooey Deschanel), a would-be stand-up comic who’s been in a long-term relationship with lawyer Cindy (Rashida Jones), but has kept a major secret from her partner involving painter Christian (Hugh Dancy). Once again he proves the vessel through which an accidental unwelcome revelation spoils a sister’s life.

A couple of other subplots thread their way through the story. One involves Ned’s eagerness to reclaim his dog Willie Nelson, which the abrasive Janet refuses to give up. The second has to do with his regular meetings with his parole officer, in whom—on his usual blundering ways—he’s apt to confide more than he should.

“Our Idiot Brother” wants us to embrace Ned as a lovable man-child whose innocent honesty can help others despite themselves. But though Rudd is a likable fellow, the character’s obtuseness is sometimes so overwhelming that the viewer may find him irritating, too. And while Banks, Deschanel and Mortimer are all good actresses, they’re stuck with playing women who are all so needy or self-centered or shrill that their sudden conversion to supportive siblings in the final reel comes off as a mere narrative convenience. The same might be said of Hahn, who makes of Janet a thoroughgoing shrew.

As for the other men in the cast, Coogan plays his role with such an unremitting snarl that he makes even the three sisters look nice (but doesn’t make Dylan funny), and Dancy is nondescript. Miller certainly proves a convincing lamebrain, but the best of the lot is Scott, who gets some of the script’s better lines. None of the actors are particularly helped by Peretz’s seemingly laissez-faire direction, by Yaron Orbach’s pedestrian camerawork, or by Andy Mondshein and Jacob Craycroft’s none too nimble editing.

“Our Idiot Brother” adds some strenuous sex scenes and a bit of nudity, presumably to secure the “R” rating that seems a near-necessity among adult-oriented comedies nowadays. But they add nothing to a movie that has a few laughs, but despite its laid-back hero has a forced, oddly strident feel.