Reikoku Koushaku wa Azarashi Hime ni Shuuchaku Chuu

Odette and Cordelia are seal sisters with the magic to transform into a human. The younger sister, Odette, explores the human world with a bright and curious heart. But Cordelia, the older sister, is her polar opposite. A painful past encounter with humans has left her wanting nothing to do with them. One day, the sisters get caught up in a dangerous seal-hunting incident. Just as all seems lost, they are saved by Rudolph, the heir to a powerful dukedom. Though Cordelia tries to keep her distance, Rudolph finds himself strangely drawn to her. Can a forbidden love between a human duke and a seal princess ever work? Cordelia's guarded heart begins to soften as they are drawn together.

https://mechacomic.jp/books/254653