The Ruby Kingdom — Patricia Bow

Mountains, travel, adventure -- Amelia Hammer wants it all. Instead, her globe-trotting parents have dumped her in tiny, boring Dunstone, Ontario, in the middle of winter with her grandmother and her geeky cousin Simon. Simon isn't having much fun either, saddled with a sulky stranger in black leather and with neon hair.And he has to be nice to her.

But life in safe little Dunstone turns dangerously exciting when Mara comes to town. Amelia is enchanted by this tall, proud, fearless girl, but Simon worries that Mara might be mixed up in something weird. He's right. Mara is not what she seems, and neither is the shape-shifting assassin who's tracking her. When the cousins take Mara's side in a war for an alien world, Amelia has to spread her wings -- literally -- and Simon must find out how far he will go to save a cousin who has become a friend.