Sennen Oukoku no Hana: Tensei Joou wa Nidome no Sei de Koi Negau

Karin is the reincarnation of the legendary queen who built peace for a thousand years. She was meant to live like an ordinary person, but when she learns that her younger brother, who was chosen as the new king, is in danger, she decides to enter the royal palace! There, she happens to meet Shion, the king of a neighboring country and the love of her life! (Source: Kadokawa, translated)