Volatile, comely ""underachiever"" Jill Gardine, 15, and icy, ungainly, brainy Caroline Hanover, 16, ""detested each other on sight""--as a weepy Jill puts it the day their respective parents, Frances Gardine and Berkley Hanover, are married. And, at the suggestion of Caro's unmaternal doctor-mother, she and her eleven-year-old brother Dean are to live with the new family. Frances and Berk, nurturing sorts, are delighted. Dean and ten-year-old Abby Gardine, both animal-lovers, hit it off instantly. But Jill and Caro (who alternate as narrators) rub each other wrong every which way. When Caro scrubs the bathroom to work off tension (their mother gave them away!), Jill, affronted, dubs her ""Ms. Clean."" When Jill inadvertently says something tactless about the color of Caro's sweater, Caro wings her with a volley of insults. Parental appeals are unavailing, and so is the cool-it advice of Jill's steady and Caro's new-found flame. Frances and Berk miserably break up. Jill and Caro, no more content, confront their guilt. Caro blurts out her feelings about her mother to an also-single-minded bio teacher. Psychodrama strings, in short, are pulled. And the resolution is a convenient crisis: Frances and Jill come down with a bad case of the flu, and Abby asks Caro for help--setting the stage for Jill and Caro to talk out their differences. But their verbal sparring, to the good-natured last, gives the book a vitality that offsets its problem-situation slickness.