In the story, it is common to be a substitute in prison, especially for nobles. My speculation on why her brother makes her the substitute in prison is- In that time, her brother is the head of the rebellion (or something like that) and he thinks that it is safer for her to stay out of their house since assassination is usual on that time. The prison is known to be so strict and is a perfect shelter/shield for her until the rebellion (?) calms down. Also, if u think life in the prison is hard for the MC, ur wrong. She finds it comfortable and even wanted to stay in there for longer time. She can also do whatever she wants since the warden loves her (romantically) and the warden is her brother's bestfriend of whom her bro tasks to protect her and let her do/get whatever she wanted.
SPOILER
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Well, at least rebelling against his father.
At some point, when Iana asks in a letter to her brother what kind of family they are, he asks her who is bothering her.
In one of her conversations with Lenag, he tells her that she isn't there because she did something wrong, but for her protection.
And when Chaser visited her in prison he said he was worried that someone (a man) touched her again.
It would be super dark if that someone is their father, but the whole story is. But it could be anyone really.
On the other hand we know the original Iana said she would never talk with her brother again. This could be because he didn't protect her against this someone, or as many other things, it could be that he is over protective of her. Imagine she had a partner and her brother didn't like it and locked her in prison to keep them apart...
The Novel's translation is on chapter 87 (that I know of), but the real information is rare.
There could be so many different stories from the little knowledge we have...
Why does this look like the brother is going to be in his sister's love triangle?