My Sister is the Main Character

“I understand that saying this so suddenly might surprise you.”
…What is he trying to say?
“I have fallen for your soul.”
…What?
The one who should be standing in front of me is the deranged male lead from a dark, captivity-themed story.
But who is this man, kneeling politely and smiling shyly?
***
When I reincarnated, I found out that my sister was the protagonist destined to be imprisoned.
My poor sister…!
“There’s no other way! If it’s come to this, I’ll take my sister’s place.”
I confidently tried to change the original plot, but…?
“I know a ring that would suit this finger perfectly. May I give it to you?”
“I refuse.”
A flirty, obsessive man is wagging his tail like a sly fox.
And on top of that…
“Duke, do you enjoy imprisoning people?”
He brazenly(?) declares his intent to lock me up.
Wait a moment. I didn’t mean I’d take my sister’s place in being imprisoned too!
I don’t want to get involved with you. Absolutely not!
“Then… maybe…”
“…”
But then…
“Loving me might be a better option, Duke.”
For some reason, I don’t hate it… Why is that?!