The greengage summer - Rumer Godden

While the Grey family is visiting the battlefields of France, their mother becomes seriously ill. Their father is far away, busy with his work as an explorer. So thirteen-year-old Cecil is left virtually alone with her brothers and sisters in a French chateau-hotel, owned by Mademoiselle Zizi. While Cecil watches from the sidelines, her beautiful older sister Joss falls in love with Eliot, the charming English gentleman who appoints himself the family's guardian. And while the greengages grow ripe and sweet in the sun, the sense of danger and mystery increases.