Sunshine Cleaning, Christine Jeffs, 91 mins, (15)My Sister's Keeper, Nick Cassavetes, 109 mins, (12A)

Adult life is rubbish in a tale of crimes and miserable cleaners

There's no shortage of films about middle-aged, middle-class men realising that adulthood hasn't turned out as spectacularly as they'd hoped (probably no surprise when you consider the lot of most screenwriters), but it's refreshing to see a film about a young working-class woman in the same position.

In Sunshine Cleaning, that woman is the ever-adorable Amy Adams (Doubt, Enchanted), whose character peaked when she was head cheerleader at high school.

Still stranded in her mid-western home town, she's now a single mother who cleans the houses of her former schoolmates. She also takes care of her lazy rock-chick sister (Emily Blunt, with a flawless American accent), and a widowed father (Alan Arkin) who sells tinned popcorn from the boot of his car.

Her love life consists of motel-room liaisons with a married policeman (Steve Zahn) who alerts her to a lucrative business opportunity: cleaning up gruesome crime scenes after the corpses have been hauled away. The job seems so ripe with possible comedy-thriller plots that as soon as Adams and Blunt start scrubbing blood stains off bathroom mirrors, you're just waiting for them to find a stash of drug money down the back of a settee.

But Megan Holley, the first-time screenwriter, resists the temptation to go down the Coen brothers route. Instead, she's fashioned a soulful comedy drama. There's no plot to speak of, but there's an abundance of small twists and turns as the film delves into its characters' lives, and reveals why it is that Adams gets such satisfaction from tidying away traces of violence and pain. Sunshine Cleaning has been promoted as a follow-up to Little Miss Sunshine, with which it shares two producers, one actor (Arkin), and half a title, but it's a more bittersweet slice of Americana than that implies.

There are a couple of cutesy quirks too many, but they're offset by Adams's touching performance. As in Junebug, which earnt her an Oscar nomination, she's better than any other actress at conveying both perky positivity and the desperation which that positivity doesn't quite conceal.

Little Miss Sunshine herself, Abigail Breslin, stars in My Sister's Keeper as an 11-year-old moppet who was conceived in a test tube so that she can donate blood and bone marrow to her leukaemia-stricken older sister.

For most of her life, this arrangement hasn't bothered her, but now that she's required to give up one of her kidneys, she marches into the office of an oily lawyer (Alec Baldwin) and instructs him to sue her parents (Cameron Diaz and Jason Patric) for "medical emancipation".

The strange thing about the film, which is adapted from Jodi Picoult's novel, is that it sets up a splendidly silly courtroom drama – handily, Diaz is a lawyer herself – and then keeps clear of the courtroom for an hour or so, while we sit through a mosaic of slow-motion flashbacks, minor-key piano ballads, and voice-overs telling us what we already know.

If you fancy having a good old sniffle as a saintly family pulls together in soft focus, then My Sister's Keeper is the corny, hankie-soaking melodrama for you. But you'll still be left with some nagging questions.

Why don't Diaz and Breslin just discuss the kidney-donation between themselves? Why doesn't the film even touch upon Breslin's feelings about having been created as a spare-part store? And if the film-makers were so wary of their dopey central gimmick, why did they include it in the first place?