sisters kept in house, suspense fiction, different rooms are different territories

[Elizabeth]

Read since 2000. These 2 sisters are prisoners in a house. There is a neighbour boy who visits over the wall They have different rooms set up as different territories eg. ship's deck. Mother shows up occasionally. One escapes. port city. Possibly in England

[Kris]

Elizabeth, is this book for teens or adults? How old are the sisters?
Story's time period?

Can you tell us more about the rooms, what they look like, and who set this up?

Why are the two sisters prisoners - by their parents?