CURRENT AFFAIRS By Barbara Raskin Random House. 270 pp. $19.95

IN the Bible there were sisters Martha and Mary. Martha was the drudge, cleaning, cooking and feeling resentful, while Mary Magdalene sat all day at the feet of Jesus merchandising her sins and generally getting media attention. A modern hard-luck sibling is Kate Millett's younger sister, Mallory, who has said she feels like she is playing "Milton Eisenhower to Kate's Ike." In Barbara Raskin's Current Affairs, the same rotten casting pairs Natalie Myers, "a serious human being" trying to "withstand the gross temptations of our society," with glamorous journalist sister Shay, who boasts of having slept with Muammar Qaddafi, Fidel Castro and Sean Connery, and whose sociopathic self-absorption creates the problem and the solution in this jubilant and witty tale of one woman's psychic make-over.

It's the weekend before the 1988 Democratic Convention, a dangerously dry summer across the U.S. In Washington, D.C., it hasn't rained for 34 days, and Natalie's 20-year marriage to Eli, a political correspondent, has shriveled up. The funds for Natalie's store-front shelter, where "pale, morose, overweight women" show up in June "dressed for a snowstorm in the Arctic," have also dried up, leaving her jobless. Into this bummer of a summer drops Shay, thoroughly tan, about to snag a millionaire third husband, on her way to cover the convention in Atlanta. Shay plans to give Dukakis explosive Iran-Contra documents (she casually filched them from a Hamptons house) that could spell political victory for the Duke and a blitz of publicity for Shay.

The "craziness Shay unleashes is as weird as the weather." Within hours of her arrival, Natalie's car is stolen, along with the purloined documents, and by the time both are recovered, Shay, along with Eli, has gone to Atlanta, leaving Natalie to face the reprisal for stealing incriminating information. Crazy-mad Colombian drug lords start shooting to kill at a woman who wants little more of life than to be good like her mom (to compensate for that horror, Shay) and to continue her unfashionable "Great Society" politics. Never mind, Natalie goes into protective custody with Shay's boyfriend and a great-looking cop in glitzy Southampton where she gets a quick makeover and makes out.

Social dedication that runs to wanting to take the local baglady home for a shower could turn cloying, but Raskin, author of Hot Flashes, serves up Natalie's life with a zinger wit and a humor-tempered awareness that is both poignant and entertaining. Returning briefly to her childhood city of Minneapolis to escape the stepped-up violence, Natalie is chagrined to discover that mother Marge is thrilled that Shay -- a child she always considered an out-of-control hellion -- is her "Fortune 500 huntress successfully landing a millionaire." Natalie feels betrayed. Is it possible that this careful, timid woman, "who sent out thank-you notes for get-well cards, who never tried out a new recipe on company or wore brand-new shoes on an important occasion," has been a secret Shay supporter all along? Is "the thrill-seeking . . . daughter more interesting to the mother than the daughter she made in her own image?" Natalie traded a candid life (hiding a botched pre-marriage abortion) so she could remain Marge's good little girl. Now, in her 40s, childless and about to be husbandless, she feels like an altruistic jerk, as boring as her mother.

THIS GHASTLY discovery provides the jolt Natalie needs to change direction and give up goodness in favor of living. The joyful (if miraculous) denouement is like a successful class-action suit on behalf of all siblings harboring a grudge. In Raskin's fictional territory women over 40 have the kind of first-class sexual liasions usually assigned only to twentysomethings.

What makes it all work is Raskin's devotion to capturing people as they really are. The hundred or so little mannerisms she assigns to a sweet child in Current Affairs, for example, are exactly the mannerisms of children that age. Even her off-hand portraits are sharp as when she nails Shay's ex-husband's conversational style: "Barney has always had a seductive hunt-and-peck way of speaking. The pauses between his words work like vacuums to suck in his listeners." Her wit might seem barbed, but it's precise and to the point and creates a satisfying intimacy.

Consuelo Saah Baehr's latest novel is "Daughters."