Yasha Ou no Saiai

Mei, the daughter of a family that exterminates malevolent supernatural beings, grew up neglected because she lacked the power to slay demons. Meanwhile, her arrogant younger brother violates a taboo, awakening Ryouga, the king of another world! Mei is left behind...

Mei finds herself in peril, but Ryouga is actually a gentle, intelligent, and handsome young man. He teaches her the value of her hidden, rare "healing power" and invites her to become his princess. In the other world ruled by Ryouga, Mei is able to use her powers and is respected, and she finds herself drawn to him, who has given her a place to belong. However, in the real world, her family has shifted the blame onto Ryouga and, unbelievably, is plotting an invasion of the other world—!?

(Source: Kadokawa, translated)