The Stone-Faced Boy — Paula Fox

There he was, Gus Oliver, searching for a stray dog in a snowstorm at four o'clock in the morning. If it hadn't been for his sister, Serena, who could get Gus to do anything, he'd be at home and warm under the army blanket in the blue room...But which, Gus wondered, was worse? The lonely darkness and the howls of that unlikable dog, or the blue room itself, crowded with nightmares, where he was supposed to sleep until Great-Aunt Hattie's mysterious visit was over? And there he was, stuck in the middle of his large family, feeling locked behind a face that didn't smile and didn't frown, and hadn't for a long time before Serena and his great-aunt got him into this mess.