Kaijin no Hanayome

Twin sisters born on a small and isolated island, ruled by the Takara family, the descendants of the sea god. Missokasu's The younger sister, Aoi, grows up in the shadow of her beautiful and popular older sister. She forms a strong connection with Ashawa the son of the Takara family.

However, both the sisters were chosen as candidates for shrine maiden of Mikojin shrine and to be the bride of Ashawa...!?

source: https://www.mangaupdates.com/series/fkochnj/kaijin-no-hanayome