The Teeth of the Gale — Joan Aiken

Kidnapped!
Felix Brooke has been summoned for a risky mission: to recover the kidnapped children of Dona Conchita, the cousin of his old friend Juana. But these are dangerous times in Spain, and the mission could be a trap. Felix warily sets off with the rescue party into the mountains, where he is reunited with Juana for the first time in five years. Felix hopes that he and Juana might become more than just friends on this journey, despite the fact that she is preparing to take her vows as a nun.

But there are more immediate troubles for Felix to deal with -- like bears, treacherous terrain, the selfish whims of Dona Conchita, and a strange man who seems to be trailing him. And when the rescue party finally arrives at the isolated castle where the children are being held captive by their allegedly insane father, there are more chilling surprises in store.