Tearjerker movies are like horror flicks in that they can yank a visceral reaction even from people who hate the genre. Nobody laughs at a bad comedy, but tender souls hide under their seats during Saw V, and cynics often admit they cried during a “weepie” like My Sister’s Keeper, though they claim they despised every manipulative second. “I couldn’t help it!” they protest. “It was about a kid with cancer!”

Here’s the thing: While real-life stories of mortality may always reduce us to mush, a fictional drama about 50 kids with cancer won’t move anyone to tears unless at least one of those kids can act. Full disclosure: I didn’t cry during this movie. But when I came close, full credit (or blame) belongs to the performances of Sofia Vassilieva as the teenager dying of leukemia, and of Joan Cusack as a judge recalling her daughter’s death. The rest of the film is itself too formless and miscalculated to wield any mush-making powers whatsoever.

The source novel by Jodi Picoult has a clever premise: Eleven-year-old Anna (Abigail Breslin) was created to keep her sister Kate alive, and now she wants to stop. Conceived via IVF as a perfectly matched donor of bone marrow and other cells, Anna has submitted to invasive medical procedures for as long as she can remember. But when Kate goes into renal failure and needs her kidney, she balks. Anna enlists a slick lawyer (Alec Baldwin) to secure her “medical emancipation” from her parents, and the girls’ iron-butterfly mother (Cameron Diaz), a lawyer herself, opposes the petition and ends up cross-examining her own daughter.

Never mind whether any of that is plausible. (Is it ever legal for parents to seize an unwilling minor child’s organ?) In the hands of director Nick Cassavetes and screenwriter Jeremy Leven, it just doesn’t work. They’ve chosen to replicate the novel’s use of multiple narrators by giving every single family member — and even, occasionally, Baldwin’s character — a voiceover. It sounds like a nice idea on paper: We get everyone’s perspective. In practice, though, this means we watch Jason Patric (as the girls’ dad) gaze soulfully at Breslin while his voice informs us, “I looked at my daughter and wondered how we got from there to here.” Or we see Evan Ellingson (as the oldest and “problem” child) sneak back into the house while narrating his fear of being caught. In short, stuff we already know because we have eyes. The narrators give the whole film a tinny, stagy feel, like one of those family-trauma reenactments on “Unsolved Mysteries.”

When the movie isn’t talking at us, it often goes into music-video mode, as the family cavorts in prematurely nostalgic slo-mo. Just when you think Cassavetes has exhausted his catalogue of lugubrious covers of songs that used to rock — wait, there’s a totally out-of-context version of the Talking Heads’ “Heaven.”

My Sister’s Keeper wastes a great subject. Ultimately, it’s less about Anna’s rights to her own body than about Kate’s right to die. Vassilieva (best known as the eldest of Patricia Arquette’s weird-looking kids on “Medium”) plays her as a young woman with enough vitality to fight like a tiger and enough wisdom to know when it’s over. She outclasses Diaz, whose performance as the stubborn mom boils down to dark eye-circles and vehemence. Diaz tries hard, but she and the girls never feel like a genuine family. It doesn’t help that they don’t look related, or that Breslin, still as cute as she was in Little Miss Sunshine, doesn’t seem to be able to nuance her perkiness in the ways this role demands.

Even the schmaltziest film can have heartbreaking moments, but they don’t always come when you’d expect. A few seconds of Cusack’s character contemplating her daughter’s death while her mouth twists uncontrollably is worth two hours of Diaz’s red-eyed ranting. At such moments, thanks to an actor who taps into something real, we remember why the world needs tearjerkers. We may never need to outwit the Jigsaw Killer, but we all have to face death.