Meshitsukai Reijou wa Kokkyou wo Koe, Tekkoku no Koushaku Kishi-sama ni Dekiaisareru

Shirley, the daughter of a frontier count, is mistreated by her family every day. When her step-sister tries to kill her, she ends up running away to the neighboring country. She isn't suppose to go into the enemy territory, which is rumored to be "vicious and cruel." But… she had nowhere else to go. At a loss in the foreign land, she receives a proposal to marry Duke Oliver, who is a knight of the enemy kingdom!

(Source: BookLive, translated)