Prisoner of Ice and Snow by Ruth Lauren

She'll do anything to break her sister out of prison--even get arrested on purpose.

When thirteen-year-old Valor is sent to jail, she couldn't be happier. Demidova's prison for criminal children is exactly where she wants to be. Valor's twin sister, Sasha, is serving a life sentence for stealing from the royal family, and Valor is going to help her escape . . . from the inside.

Never mind that no one has escaped the prison in centuries. Valor has a master plan and resources most people could only dream about. But she didn't count on having to outsmart both the guards and her fellow prisoners. If Valor's plan is to succeed, she'll need to make some unlikely allies. And if the plan fails, she and Sasha could end up with fates worse than prison.