I thought her character wasn't mentioned in the original book?
No her character is mentioned just not gone into, she knows that he had a sister who dies because the other lead with the brown hair was said to have been in love with her. I can understand that she didn't know her identity initially, but when she figured it out why is she asking why he's acting differently from the book? Well in the book YOU DIE. You know the sister died because you had a moment that was like "When I entered her body I could tell that she had died" GIRL KEEP UP (not you, the mc)
When she was having a convo with the other prisoners they were like "Oh and the duke has an alleged younger sister". So she should have figured out when she learned it was her
1)oh I'm the duke's younger sister
2) She's supposed to originally be dead
3) He loves his younger sister dearly (from the letters and the gifts)
4) No other known living relatives
OBVIOUSLY YOUR SUPPOSED DEATH WAS THE CATALYST
"Omg he didn't act like this in the book" Well if we remember correctly from the book YOUR CHARACTER FUCKING DIES.