Don't worry though, it has a happy ending.

I also absolutely love how you can see the gradual development of MC's and ML's desire for vengeance during their imprisonment. At first neither MC nor even the reader can imagine why the siblings would ever grow up to be villains after initially being such sweet and innocent children. In fact MC's primary motivation at the beginning is to keep the two brothers from becoming evil. But as the children are continuously ab*sed/neglected to the point that Estelle is constantly on the verge of death and Lettis becomes suicidal, MC's and ML's hatred for their captors grows more and more, until eventually MC promises that she'll kill them herself once she and her siblings escape. The shift from "let's be innocent, good and morally upright people" to "let's destroy them all and make them pay" is so organic and believable.

honestly I adore how dark a turn the story took when you start realizing just how different the children became over time. Lettis become more temperamental and depressed instead of his originally kind and nagging big bro personality.

Venya who was once mischievous and c*cky become quiet and withdrawn, only trusting Sasha. Estelle who they all loved, became less of a preteen girl and more of a young child trying to not be a burden.

Then Sasha, poor Sasha. She did her best to keep Venya and Lettis from blackening and tried so hard to protect Estelle. She didn't want the boys to become villains or any of them to suffer and try to get revenge. In the end, she became the driving force for getting revenge, the exact opposite of what she originally wanted. She is such a well written out heroine and her own blackening was so sympathetic and understandable, she set the tone for the plot so well.

I cried when Estelle died *sob* the poor child