Responses
I'd read the heck out of that!
Oooh! And it would be awesome if she was isekaied the same time our mc was, so she lived there her whole life, but was also from our world... I'd personally also make it so that the grand Duchess hasn't read the book and thought she had just been isekaied into some random fantasy world, but she LOVED this era, so she got really into running a ducal estate.
Ml's sister could have been another manhwa's mc with her intelligence and beauty.
Ml's sister x mc's brother would a good combination