Sapphire in the Sand — Patricia Robins

Emma is sixteen years old and in love with Andrew, but he;s engaged to
Julie, Emma's beautiful elder sister. When their two families go to
France on holiday together, Emma is perturbed by Julie's casual attitude
to the engagement and her interest in Yves Courtelle, another guest at
the hotel.

Emotions become hopelessly entangled, and for the four young people, the
holiday threatens to end in tragedy as they struggle with feelings which
are stronger than they are.

A compelling classic romance from the inimitable Patricia Robins, first
published in 1967 and now available for the first time in eBook.