Keeping a maudlin aura at bay, "My Sister's Keeper" defies genre expectations somewhat and merges tough with bittersweet, even humorous when appropriate.

Based on the popular book, the film doesn't insist you like any of its distinct characters, and certainly few of them are perfect. That reality base, though, helps keep the film's integrity intact.

Cameron Diaz and Jason Patric play Sara and Brian, parents of three children. Kate (Sofia Vassilieva), the middle child, has been ill since she was small, living into her teen years despite leukemia. One of the keys to her survival is her younger sister Anna, played by Abigail Breslin, a test-tube baby who was created specifically to help her sister survive by donating blood and other essentials that matched her sister's. Years of such donations, including painful procedures and surgeries, have persuaded the now-teen Anna to defy her mother's orders and stop giving to her sister, especially now that a potential kidney transplant is involved.

Anna hires a popular lawyer seen on television commercials to sue her parents and win her early freedom as a minor so she can stop the process. This doesn't sit well with the tough, almost emotionless Sara, a lawyer herself, who defends her case in court at the same time she strongly urges her youngest daughter to understand what this defiance means to the family.

Along the way, the sisters grow even closer as Anna stands up for herself, and when it's apparent Kate doesn't have much longer, the family makes a desperate attempt to bond and gain perspective before it's too late.

"My Sister's Keeper" is a weeper with plenty of tough talk, dark humor and an unflinching way of presenting terminal illness in truthful terms. The story allows each character to present unexpected sides to their personality and deeper motivation. It ably examines married couple, mother-daughter, father-daughter and sibling dynamics with reasonably intelligent and realistic dialogue. An abundance of fresh, offbeat music prevents "My Sister's Keeper" from veering toward cliche. The film is unafraid to make you cry, but doesn't rely on many of the standard trip-wires of this genre. It's tricky that way, though neither is it a groundbreaking, completely surprising approach.

The film allows Diaz one of her first real grown-up roles, far from glamorous, and she is supported by Patric's grounded presence. Breslin is good, again, but for performance, this is Vassilieva's spotlight, and she commands attention with her wonderful, varied-tone acting, keeping "My Sister's Keeper" steady.

*** (out of four)

"My Sister's Keeper"

Starring: Cameron Diaz, Abigail Breslin, Sofia Vassilieva, Jason Patric, Alec Baldwin

Director: Nick Cassavettes

Studio: Warner Bros. Films

Rated: PG-13 (for mature thematic content, some disturbing images, sensuality, language and brief teen drinking)

Running time: 112 minutes