The Children of the King by Sonya Hartnett J Har

Cecily, 12, and  her 14-year-old brother Jeremy  board a train in London, heading north, along with other evacuees during World War II. But they are not like the other children being sent out of harm's way. Cecily and Jeremy's mother is accompanying them, and they will be staying with their father's brother in the old family mansion, Heron Hall.
Cecily, always one to feel inferior around Jeremy (he's older, smarter, more passionate) feels sorry for the children leaving the train at the village and being shepherded into the town hall to await selection by a local family. She thinks it would be a noble thing to take in one of the evacuees -- and get a playmate for herself. To her surprise, Jeremy agrees, and their mother is happy to let Cecily make the choice. That's how May Bright ends up at Heron Hall.
May is adventurous where Cecily is not, but Cecily is determined that they be friends, so she traipses after May across the lawn, through the woods and over a stream, where they discover the remains of an old castle, tucked away where no one has seen it for years. And much to their surprise, they discover two boys who are living there. But the boys aren't there every time the girls come visiting.
Cecily asks her uncle about the castle, named Snow Castle. He begins telling the story of the castle, a little bit every night before bedtime. But first, his audience must understand a little bit of England's history from centuries past, including an evil duke who kidnapped the rightful heir to the throne, and made himself king instead.
This was an enjoyable book to read. It wasn't fast-paced or one that made me want to turn page after page. It was just a nice story that unfolded slowly but kept my interest. If you like books set in another time, or enjoy stories about royal families, give this one a try. You might get a surprise at the end.