Sometimes I feel we're just four people who were put together by mistake,"" confides 15-year-old Delphine, who's talking about her family while summering with divorced mom Nicole and younger sister Amanda on the sun/sea paradise of Greek isle Mykonos. All her life Delphine has suffered the chill of a child shut out, unplanned and unwanted by selfish, vain Nicole and career-absorbed dad David. As a baby she became the responsibility of Helga, the stolid, ""muscle-bound"" nurse, while Amanda was the wanted baby, loved and cuddled. What now finally breaks up this logjam of self-pity in Delphine's heart is a week's idyll with teenage Simon on the near-deserted island of Delos: ""they swam, made love, lay in the sun. . . flung themselves into the midnight sea."" And at summer's end, Delphine, having given and taken freely, is at last able to care about Nicole, who has been through a series of lovers and is now stunned by David's remarriage; and Delphine can even appreciate Amanda. All three are ""evolving"" women. . . ""thorny . . . but themselves."" Thorny indeed, and though Warburton does sensitively touch upon the helplessness of small children at the mercy of adult caprice, the Mykonos atmosphere is heavy rather than heady and the prose is over-creamy.