In the saucy short profiles she regularly turns out for the New York Times, Witchel displays a knack for breeziness that doesn't come so naturally here: Sustaining it for the length of a book means sometimes forcing it, and this time the subject is personal--the bonds and unresolved tensions among her mother, her sister, and herself. Alex is the ``middle of the perfection sandwich''; more, more, more would please Mommy (a.k.a. Wonder Woman); less would be easier on Phoebe, ten years Alex's junior, who would then ``have only one Supreme Being to worry about'' (Mommy). Apparently, it's not so cushy being the eldest (of four), the high achiever, the responsible one--and the one who had to mother the others while Wonder Woman (impelled by childhood polio to overcompensate) got her Ph.D. and went to work when nobody else in Scarsdale did. Which is why Alex chooses, adamantly, not to have kids of her own; besides, her plate is full enough, thank you, with work, husband (finally, at 33), plus two stepsons on the weekends. Desperate to become herself, Alex is up-front about needing to Separate from ``the human Swiss Army knife'' who can do it all--yet she keeps seeking Mommy out, seducing her with article-generating junkets (to a trendy motel in the Hamptons, the deluxe Stanhope in Manhattan, the home of simpatico actress June Havoc). Although she never owns up to resentment of Mommy, other angers come close to the slick surface: at Phoebe for coexisting in the Alex/Mommy world; at the nobler-than-thou friends who martyr their tired selves to rear their little geniuses (whereas, Alex shrugs almost redeemingly, she'd probably just have a regular baby). Women who see themselves in Witchel's mirror (``pushing middle age and wearing the same clothes I took my SATs in'') may be willing to give her the benefit of the doubts raised by her presumption of an audience for all of her privileged communications.