Melusine — Lynne Reid Banks

What is the sinister mystery of Chateau Bois Serpe? Roger, his parents, and twin sisters arrive at this crumbling ruin in the wilds of France, anticipating a holiday that's "different." And so it proves, but not as they'd hoped. The owner, the strange daughter Mesuline, whom Roger first meets tending her father's goats. She is like no girl he has ever seen, with her brusqueness and her expressionless black eyes, and as he gradually befriends her, he begins to realize that there is something very wrong about her, about the chateau, and about her life within it. Roger senses a cry for help. How can he know that by trying to answer it, he will involve himself and his family in a shocking drama that reaches its climax in a grisly discovery -- and sudden death?