Angels Flying Slowly — Jill Roe

When their father runs off with another woman, Isobel and Caro are left to their mother, Iris, who relieves her frustration with bouts of fury directed at her daughters. Bewildered and hurt by their mother's lack of affection, the girls turn inward for support, erecting a fence of fierce mutual loyalty against the outside world.
Dispatched to a convent boarding school, the sisters find the discipline harsh and the mysteries of religion alien. Although this community of women is rife with petty rivalries and unhealthy passions, they also discover a love and security lacking at home, until the arrival of their cousin Ursula.
Exotic, beautiful, and obedient, Ursula is everything the sisters are not. Abandoned by her own mother, Ursula insinuates herself into Iris's affections and heightens an already tense family situation. At the convent, Ursula's knowing gaze provokes nuns and girls alike. She perpetrates sly tricks everywhere and always escapes punishment. Pushed to the limit, Isobel decides to exact her own revenge.