Reckless , av Cornelia Funke. Pojken har faktiskt en bror, inte en syster: flickan är brors flickvän.
Från Wikipedia:
Soon after Will arrives, he is attacked and scratched by a Goyl, a humanoid race with stone skin. As a result of the attack, Will's skin begins turning to stone.
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With time running out, Jacob, Will, Clara and Jacob's vixen friend, Fox, journey to the witch's house ...
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... on their mission to rescue Will, they travel cross country to the mountains underneath which the Goyl have built their capital city.