Koukyuuhi wa Ryuujin no Ikenie Hanayome - Goshinsenzan Monogatari

Renka grew up being compared to her talented older sister and being oppressed by her parents. She enters the inner palace, but due to her sister's tricks, she is chosen as a terrifying dragon god's sacrificial bride. At the depths of despair, she headed to the mountains, where she was greeted by a handsome man named Seiran. The communal life that started with a time limit, but...?