Koishita Hito Wa, Imouto No Kawari Ni Shindekure To Itta summary:

In place of her sister, Wisteria is a former daughter who has become a guardian of a poison-filled otherworld. One day, a man who resembles her first love appears from the sky. His name is Lloyd. To her surprise, he is the son of her first love!As a proof of his marriage proposal to the princess, he has come to ask for Wisteria's partner, Sartis, a holy sword that can understand words. Having survived in a frozen time, she tells him to defeat Lloyd with her own magic and return to her own world. I will take you as my apprentice so that I can defeat you! But, despite his efforts to persuade her, she is forced to stay. The relationship between master and apprentice begins one-sidedly, with the apprentice learning magic as she goes along. Will her time, which has remained frozen, begin to move again?A master-student love fantasy between a lonely former daughter and a genius nobleman!