The starbuster - Berlie Doherty

At first, Tam isn't at all keen on his baby sister, Blue. All she seems to do is cry. But with a little inventive persuasion from Great-grandpa Tam changes his mind and even goes as far as to admit he might quite like her. But then one terrible night, Blue is stolen and an ugly fairy changeling is left in her place. With his Mum and Dad wrapped up in grief, and his Great-grandpa far too old for adventures, it falls to Tam to venture to the land of Faery, return the changeling and win back Blue. But it is a hazardous journey, and Tam must keep his wits about him as he crosses the Sapphire lake, does battle with the castle nightmares and finally confronts the King and Queen of the fairies.