Julia Roberts, Susan Sarandon, ...

In the beginning, “Stepmom” plays like a broad catfight sitcom in which Jackie (Susan Sarandon), who got the large tasteful suburban house and joint custody of the kids in the divorce settlement, regularly bad-mouths and sabotages Isabel (Julia Roberts), the groovy younger career woman who shares a downtown-loft bed with Luke (Ed Harris), Jackie’s ex. Some catfight. Some broads. Isabel is a fashion photographer who demonstrates she’s brilliant at her job by snapping the shutter three times, then declaring the assignment done, and shows that she’s terrible with children by oversleeping when she should be getting angry 12-year-old Anna (Jena Malone) and scampish 7-year-old Ben (Liam Aiken) ready for school. The kids enjoy giving Isabel a hard time, partly because they’re loyal to their mother (“Mommy, if you want me to hate her I will,” Ben offers) and partly, I assume, because they hate that their father’s new girlfriend owns so many cute hats.

Director Chris Columbus, whose talent for creating decorative gingerbread-house domestic comedies benefited “Home Alone” and “Mrs. Doubtfire,” takes an awful lot of pleasure in the girl-girl competition, encouraging the worst in Sarandon and Roberts: Older and younger movie star snipe and glare at each other with little subtlety, and little chemistry either. The two characters appear to be skirmishing only because they’re supposed to by convention; left unbothered by a speechy, pronouncement-filled script it took five writers to assemble, they probably would have little use for each other.

“Stepmom” takes a serious turn for the mawkish, though, when Jackie is hit with cancer — what kind we don’t know, only that “it’s spread” and that’s very, very bad. (“But I did everything right!” she wails.) The movie trailer lets on nothing about this arbitrary, very, very sad development, I know. But it’s important to report, at the very least to help ticket buyers decide whether another dying-mom tear tweezer — coming so soon after motherly Meryl Streep sickened in “One True Thing,” and arriving 15 years after motherly Debra Winger expired during the holiday release of “Terms of Endearment” — fits in with year-end entertainment plans.

Even moviegoers looking for catharsis, meanwhile, may well question why “Stepmom” requires Jackie to hide the state of her health from her kids or her ex for so long. That selfish action is an unlikely F— you to the people she loves, engineered entirely (and unfairly) so that Isabel can be the one to discover the truth and ease the way for girl-girl reconciliation. (“You’re Mother Earth incarnate!” Isabel sighs. “I have the children’s past and you have their future,” Jackie recites.) By then it’s Christmastime, and while she looks nowhere near death, Jackie more or less bids a fond farewell to everyone, probably so she won’t have to put on white powder and red eye shadow like Streep did recently in a movie that, compared with this lab-engineered weepie, is the soul of realism.

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