Children's fantasy book involving a character called Cassiopeia

[Aspen]

The book had a young female main character, I can't remember her name.
I don't remember much about the plot, but some of the themes were
- Some sort of fortune or a large amount of money or gold
- A bad father - I think he tried to determine the MC's future
- MC was locked up at one point
- Definitely a woman named Cassiopeia, she may have been a pirate?
- I think there was a pirate ship and an island
- A vaguely historic time period (sorry)

I read the book around 2014-2016, but it was from a school library so may have been published a while before that. It was presumably published in the UK and was written in English.

I know this isn't much detail but I'm hoping this might spark a memory and maybe a title in someone! If you have a book that doesn't fit all the details please tell me anyway as I might have remembered something wrong. Thanks in advance!

[Sarah]

I don't think this is what you are looking for, but maybe Pirates! by Celia Rees?

Other things that might help, how old were you when you read it? Roughly how old was the MC?