Bernstein ( The Flight of the Stork ) here provides a needed book on mixed-family households. The author is a family therapist in Berkeley, Calif., also a stepmother of two and mother of "ours," the child born to her and her husband. In a well-organized and admirably clear text, she combines material on her own home life with interviews with parents, steps and half-siblings. Humanizing bald statistics, Bernstein creates awareness of how divorce affects today's children and those responsible for their care. Sustaining harmony in households made up of offspring of previous and current marriages can be a trial, notes Bernstein, whose empathetic counsel should prove helpful. Her graceful revelations of "fragile and interchangeable families" will have wide appeal.