Sepulchre (Languedoc #2) by Kate Mosse

In 1891, young Léonie Vernier and her brother Anatole arrive in the beautiful town of Rennes-les-Bains, in southwest France. They've come at the invitation of their widowed aunt, whose mountain estate, Domain de la Cade, is famous in the region. But it soon becomes clear that their aunt Isolde-and the Domain-are not what Léonie had imagined.