The Intruders - E.E. Richardson

Joel Demetrius is quite looking forward to moving in with his new step-family, but as far as his sister Cassie's concerned, they're nothing but intruders. She doesn't want anything to do with Gerald and his two sons, and to make matters worse their new home is a derelict old house, neglected for decades. Joel thinks it's interesting. Cassie thinks it's a dump.