Grace Livingston Hill 
The Ransom 

It was their stepmother’s death that brought Christobel Kershaw and her younger brother Randall home at last. But the large elaborate mansion chosen by their deceased stepmother didn’t really seem like home. And their father seemed different too—so tired, so weighed down. Christobel wondered if there was any way they could help him. Could they start over again and become a close, loving family?
Slowly they learned to trust and love each other. Then, just when it seemed they were indeed becoming a real family, disaster struck! Randall—young, impetuous, Randall who was just beginning to understand what true manhood meant—was kidnapped. Would they be able to come up with the ransom? And would they be in time?