- Orphans or de facto orphans who have to largely fend for themselves, check.
- Said almost-orphans live in genteel poverty, check.
- Mentally ill father or father figure, check.
- Isolated medieval castle, check.
- 1930s setting, check.
- Teenage narrator who narrates by writing in her journal, check.
- "Plain" narrator and more beautiful older sister/cousin, check.
- Love interest named Simon, check.
There are, of course, some differences, the major one being that A Brief History of Montmaray addresses politics and the impending world war whereas those go unmentioned in I Capture the Castle. A Brief History of Montmaray is also the first book in a trilogy.
Can you think of any other two books with so many similarities? I wrote about others that came to mind here, particularly Roald Dahl's classic Charlie and the Chocolate Factory and Wendy Mass's recent book, The Candymakers and here.