ReachBish

I read a manhwa but i forgot the title.

Plot goes like this:
She's transmigrated to a novel. She's a side character and the sister of the villain. She's barely mentioned in the novel.

The main story hasnt started yet at this time.

She's in prison for the nobles.

The original ML is caged in a dungeon there. The ML is cursed. He acts like a dog and sometimes comes back to his timid self.

The girl keeps mentioning how the novel is R18.

The warden of the prison has a crush on her secretely.

Her brother sends her a blank letter each month so that she could right what she wants and then he will ask someone to give those things to her in prison.

Thanks in advance (⌒▽⌒)