Wavering in tone between black comedy and slick soap, novelist-memoirist Johnson (World of Henry Orient, Flashbacks) offers the contrasting lives of estranged, 42 year-old twin sisters--as presented in their alternating chunks of narration. Cassie Armstrong Coopersmith leads off, telling us about her super life in Southern California--with a TV-producer hubby, three kids, a Naugahyde ""playpen,"" a Betamax, and a just-too-perfect house in Santa Barbara; Cassie is fastidious, religious, banal, complacent, priggish, not very bright; the voice--funny for a while, then fairly tiresome--is something like Carol Burnett doing a takeoff on Mary Tyler Moore in Ordinary People, with a soupcon of Patrick Dennis' Little Me (when Cassie recalls her early acting career, defending her virtue against Hollywood perverts). Next it's the turn of twin-sister Celia Armstrong, a bright, sarcastic, sloppy, depressed N.Y.C. photographer: childless, divorced, bitter, scornful/jealous of Cassie (who was always Mom's favorite), involved in an affair with a married man, pinching pennies. And then the two sisters come together after years and years--when their father Caspar, back in Cozy Connecticut, announces that he wants a divorce after 45 years from mother Dorothy: he wants to marry his longtime mistress Yolande, a small but tough prison matron. The sisters see this situation differently, of course: Cassie is entirely outraged on Mom's behalf, Cella's more inclined to see Dad's side of things--including the subtle oppression of life with sexless Dorothy. But in a little while, as disasters heap up around them (Celia's lover betrays her, Cassie's whole family descends into a drug-related stupor back in Santa Barbara), the twins laugh and cry together a little--not very convincingly. And implausibilities mount in the chaotic final chapters: a few frenzied sexual encounters; a couple of sudden deaths; the reblossoming of divorcee Dorothy; Cassie's retreat to a nunnery (""I'm not supposed to think about it, but I can't help thinking how flattering the cream-colored habit is""); and Celia's discovery of the sunny side of life. Throughout, in fact, Johnson has problems in integrating the different sorts of fiction here--satire (heavyhanded), twin-psychology (superficial), realistic domestic/sexual melodrama (drab, vulgar), and cartoon-fable. The results are never amusing enough to suspend disbelief nor lifelike enough to command sympathy. Still, with a steady supply of comic energy and surface raunchiness, this should attract a certain semi-hip audience (which will congratulate itself on feeling superior to Cassie)--and the dollops of sentimentality may manage to soften Johnson's ugly overall portrait of family/sexual relationships.