I Reincarnated As A Villainess, But Why Did I Become A Cat Butler Instead?

I reincarnated as Rowaine, the villainess who was fated to be executed for the sin of mistreating a shapeshifter. To escape death, I decided to replace my sister, Renee, in her arranged marriage.
The problem was that the person I was to be wed to, the notoriously cold-blooded Duke Blois, was a cat person.
“Darling, I heard that abusing shapeshifters like me is a hobby of yours.”
“From what I understand, there was no such condition that a woman who mistreats shapeshifters could never be the Duke’s marriage partner.”
His emerald eyes flashed dangerously. I quickly added before he could say anything else.
“If my hobby will pose a problem, then I promise that I will never play with the Duke.”
“Ha?”
This is the story of a 10-year contract marriage between a cat butler and a feisty cat shapeshifter